SM Pro PR8 Mic Preamps

[PRR]

> The so-called "rip-off" here was just the idea of 8 mic pres in 1RU for $99.

As usual, I don't know what we are talking about (I don't read every page of the the SamAsh flyers).

8 preamps for $99 is commercially exciting but technically frightening. Those look like darn good preamps considering they retail for ten bucks each (plus $19 for power and case). I've seen far worse.

In fact some of the puzzling details make sense now: they could have got similar performance with a different and simpler layout, but new layout means design cost and time. They probably had an existing hole pattern for a "better" preamp, stamped 8 of them, and left off the expensive bits.

> I don't know why they bothered with Q1 Q2...

Because (I wonder) it was cheaper to use the existing hole-pattern (from an existing better preamp) and stuff cheap transistors than to do a new layout? Heck, if they buy the "good" transistors (which may not be expensive) in large quantity, it is cheaper to use those than to keep a lesser part in inventory.

Also the 20KHz roll-off makes perfect sense: it doesn't suck but it isn't as shimmery as their high-price (and higher profit) preamps, so it doesn't steal sales (as much).

> Input stage Q1,Q2,U2A has a gain of ((8K2+8K2)/((4K7+4K7)||4K7))+1= 6.2

It may be clearer as ((8K2+8K2+((4K7+4K7)||4K7))) / ((4K7+4K7)||4K7))= 6.2 (so you see where the "+1" came from).

> the top half of the pot (at points less than max) will actually have an additional effect on the gain of U2B as well, eh?

Yeah, but I wasn't going to worry about the control taper. The loading may be "right", or they may have let it be "a little wrong", jumpy, again so as not to embarass their $100/channel models.

> Cross-coupled output stages like U1A,U1B suck.

I do not trust those things. On-paper, or in app-notes, "you can show" that it self-compensates for unbalanced loads, even delivering the same level when one output is shorted. But some of these things are unstable in real life with odd loads. Perfect cross-coupling has to be very exact. This one does not look "perfect" to me, and probably nowhere near instability. I just don't see what the 39K resistors do except confuse things.

> "...gain control from -20dB to +40dB"

Then assuming my rough analysis of the output stage and 1.2 gain was right, and there is another dB or so of error from counting on my fingers (or marketing rounding), that is exact.

What is the use of -20dB gain on a box with 1.3V input overload? (I know: marketing.)

> I prefer not to blow up my gear, no matter how cheap.

Actual smoke is quite unlikely. The cap reduces DC gain to unity (actually about 1.5) and minimizes DC offset. With the 4K7+100 resistors and no C100 the DC gain is 6.2. A random pair of transistors from the same factory reel will have offset voltage around 10mV max. So without the cap there could be 62mV offset at U2A output, blocked by the output cap, quite negligible. If they used two utterly different transistors it could be 600mV, still not enough to reduce headroom a whole dB or strain anything. I don't think the cap does any good at all. And 470uFd is not a negligible expense in a $10 preamp. The only excuse I see (beside oversight) is that they build the SAME board for low-fixed-gain AND for vari-gain products, don't decide which it will be until final case assembly, and then either wire up a pot or stuff a 4K7.

Ya know: if you can use it full-up, 40dB gain, the noise may be better than the usual chips. Several dB lower than a pair of 5534. And as cheap as the 5532 has got, a pair of switch transistor may still be cheaper.

Oh, wait. The noise of the first stage is dominated by the 4K7+100. Noise is 5.6 times higher than a 150 ohm resistor, 15 dB Noise Figure! Saying it that way, it is hard to believe it is usable. But it goes to show that the quest for ever-lower noise figures is often futle specsmanship, and quite high NFs can be perfectly usable for many situations.

I may be wrong about the noise of the 2nd stage, U2B. At mid-rotation the high pot impedance reduces the noise gain. So the 2nd stage may not really dominate until you turn down 6 or 12 dB (34 or 28dB total gain). If the input referenced noise stays within a few dB of theoretical all the way to 30 dB gain, that is not bad.

Also the noise figure becomes moot if you use the modern condensers. Their self-noise is 10-20dB above the noise of a 150 ohm resistor, so an input with a Noise Figure as bad as 6dB or more isn't a real problem. (These mikes are not "noisy" because their output level is 10-20dB higher than a passive dynamic mike.) Although, in the type of situations where you would use 8-for-$99 preamps with $69 condensers, you might be better served by a simple unbalanced (but Phantomed) input and one op-amp (plus inverter for balanced out).

If you want a better noise figure, use a 3-way switch to bridge 1K2 or 100 ohms across the 4K7 cross-emitter resistor, and reduce the value of the feedback resistor around U2B to maybe 1/10th the present value. With the input stage working at gain of 16 or 80, 2nd stage noise becomes a non-issue. And first stage noise goes down with that resistor's value. At 100 ohms (plus the existing 100 ohms) the NF will be just over 3dB, which is good enough for most real needs.

At $99, another trick arises: tack 10K resistors to one of the output pins on the first 7 channels, bus the other end together, and rig a switch to feed that bus into the 8th channel. Walla: basic 7-input mike mixer for $99. Got a seminar conference with a dozen people on stage all talking in mikes, and all you got is an 8-in mixer? Buy a couple of these $99 boxes and lash them up as a 15-in mono mixer. Cheap chaos.

 

[hairyandy]

Sorry about the huge pics guys! I put them up without checking them out first after Charlie emailed them to me. Always check the pics first, lesson learned... :green:

 

[SonsOfThunder]

Thanks Andy, and apologies again to everyone who had to scroll sideways.

 

[Kev]

Great stuff PRR ... almost to the point of a rant. Excellent work.

It is hard to have a genuine new idea in this business and most of us have had a thought of a product that has already been done before AND is likely to be done again, soon.

These cheap $99 may not be the top of the tree but I still find it amazing that an operating unit can be brought to the shop at this price. Postage and delivery must be a large part of the cost. The instruction book and the IEC cable ... even the registration card are all still in there.

As was mentioned above ... with the addition of some 10 K resistors you can turn this into a simple mixer for a conference and so, solve a problem fast and easy.
By thinking outside the circle, many of these cheap boxes can have life slightly outside of the original intent.

It's a great time to be in audio production.
... and video production is catching up fast.

 

[SonsOfThunder]

with the addition of some 10 K resistors you can turn this into a simple mixer

If you don't need the mic pres, they already have what you need here. They have several cool items for just a few bucks. Even the OC8 (8 ch opto comp) is only USD$500, though I haven't used one. :sad: I'm surprised you haven't seen this stuff before Kev, they are located in Oz.

As I've said before, I like to find things that are decent and cheap, and then find the shortcoming and fix that. Its just a lazy way about DIY, though having an AP2 to measure on is a distinct advantage!

Happy, Happy DIY!!
Charlie

 

[AP]

Regarding the schematic posted in this thread (rather than the OT one I got carried away with earlier), my take is as follows (hopefully I can add at least something to PRRs rather excellent and learned account).

The 4k7 resistor between the emitters would certainly have started life as a 4k7 rev log pot for gain control - the 100R then serving to set the maximum gain of the the stage. Used like this, the arrangement is a pretty reasonable mic amp, with good noise figures, close to theoretical minimums...

Alternatively, you would quite often see -ve f/b from the output of U2A (which often gets to be two op-amps - doubled-up to get two identical feedback paths of the opposite phase, for balance) back to the emitters of Q1 & Q2 - even though it then looks rather different, it (gain setting etc) works pretty much the same, with the advantage that Q1 & Q2 are within a -ve f/b loop, which hopefully helps to reduce distortion... The complication then is that you end up with two separate routes for DC -ve f/b, which can end up fighting each other... so you then have to... (etc etc)

Anyway, as it is shown (with fixed gain), C100 is quite redundant, as its real purpose is to remove any slight imbalance that would cause even a tiny amount of DC across a 4k7 (variable gain) pot, which would make it awfully scratchy at the high gain end. The high value of C100 relates to the fact that it will work with the 100R to cause low-end roll off at highest gain. Basically, one would try hard to throw C100 out (but it is not simple if you want a pot there).

My take is that, unless you can actually see the footprint for a gain pot on the pcb, or holes to wire one in, I won't buy (in the nicest possible way) PRRs suggestion that they re-used an existing hole-pattern - my view is that it was (ahem) 'implemented' by someone who didn't actually understand what it was they were copying^H^H^H^H^H^H^H^H stitching together. Otherwise, why have both the 4k7 AND the 100R (the latter would effectively be swamped out by the former)?

The caps around the input are an attempt to control RF (so this is not altogether the first time this arrangement has been used by these people - they used it enough to realise that RF was a real problem). All those (comparatively) big C's are an ugly way to go about it though - some ferrites and some smaller caps would be better.

I'd say that C8 was necessary, as there is likely to be a significant DC offset on the output of U2A (which would be apparent as pot R27 was operated. But what is R24 doing ther? You amplify the signal, only to knock it down again prior to putting it through a pot, then amplify it up again? Whacky, or what?

In particular, the highest gain (and hence highest noise) in U2B (as a result of the resistor arrangement around it), is when the pot R27 has the input signal attenuated (notionally) to zero! This is surely just about the poorest possible design approach? If the guy really wanted an input resistor of 2k+4k7, then leaving out r24, and setting R25 to be (say) 6k8, would have been very much better. As it stands, he gets a max signal gain from U2B of 14.9x, with a noise contribution (at min level setting on the pot) of a 50x gain stage... That is really naff (a technical term!)

Regarding the balanced output circuit - I quite like this kind of arrangement (when done properly). Usually, you do need something to ensure that, in a no-load condition, both outputs are doing the right thing - ie. swinging equal and opposite. In practice, small variations in resistor values conspire against you, even when using 1% types, so you often need a preset somewhere to trim the output signals to be equal.

As an alternative, moving the cross coupling points (RH ends of the 39ks) to the other side of the output resistors (47R), with some kind of pre-load (say, 4k7) to ground there, would probably help a lot. A lower value than 4k7 would be better, but would start to steal useful current from the potential external load. Such a pre-load would also be a good place for a balancing preset - the actual 'center-tap' of such a load could be the wiper of a pot.

Also, I would have expected the 39k resistors to both be 10k - I'm sure that being 39k does actually screw something up (another technical term), but the circuit is a little difficult to model mentally, and I can't quite figure out what it will do, without looking for, and finding, my pencil.

So on balance, my feeling is that someone, somewhere, knew what they were doing - but not the chap who produced this circuit.

Alan

 

[PRR]

> someone, somewhere, knew what they were doing....

Yes, but there is nothing novel there, and similar or identical plans (with proper gain-set) are everywhere. Someone pointed to Rod Elliot's plan, which isn't exactly the same, but a good search might turn up a published plan with all the small details.

> ...- but not the chap who produced this circuit.

It certainly is odd. I was trying to figure the "best" reason for it to come out that way, without saying "stupid negligence". Design for high-volume low-skill mass production sometimes favors odd things.

But.... at this feature/price point, with the right promotion (bundled with an 8-in sound card and groovy software), this box could sell like hotcakes. And even if production economics and time-to-market pressure favored re-use of an existing layout, those unnecessary parts will quickly become a liability. C100 is not cheap and there are 8 per box. Say that 8*C100= $2 per box, off a $99 retail price: that's a big drag on the profit structure. Or estimate that they sell 10,000 of these, 10,000*$2= $20K, an amount that would easily(?) buy design-time and layout costs for a slightly cheaper and significantly better design. Even if the cost of C100 is only $1 per box and they only sell 500 boxes, they could have bought an hour of an experienced designer's time for a quick look-over. If s/he only said "replace C100 with a short" they would have come out ahead. In two hours, any experienced audio designer could have removed several more $0.02 parts and done a quarter-assed optimization of the rest, on the existing topology and hole-pattern.

But the performance bottleneck is the lack of a gain-set in the input, plus the fact that modern studio work involves hot mikes and loud drums. And at this price point, it may often be used in non-silent rooms (garages and basements). It is probably "better" to have NO clipping on high levels than to get a low-low noise figure.

And one could argue that there are some carefully-choosen "good" points: the same company must sell a high-price preamp which isn't much more expensive to produce. They do not want their low-price model to be "essentially as good as" their more profitable lines. So add a little noise, lose a little treble.

Except.... SM Pro Audio does not seem to have a "high-price" line. And their site claims "Our staff are dedicated audio professionals and design engineers who have a passion for product research, and are committed to bringing outstanding products to market."

 

[SonsOfThunder]

[quote author="PRR"]I was trying to figure the "best" reason for it to come out that way[/quote]
There are always things that I see in consumer audio products that cause me to ask this sort of question, though mine is usually along the lines of: If they had just spent a couple more dollars on this part...?

I can tell you that based on emails with the product manager (who seems to be a very nice fellow!) that I really don't think that they use this layout for anything else. You probably cannot tell from the pic I've posted, but the inputs (U2, etc.) are all on one PCB and the outputs (U1 and etc.) and power supply are all on one PCB. This probably allows them to use the same output and PS PCB for the PR8 mic pre and the DI8 box (I'm taking a guess...). I think they used the fixed resistor because they didn't want to buy (special order) a 4.7k reverse log taper pot. The 50k log was probably cheeper and very very easy to get. It was probably path of least resistance in the parts procurement.

[quote author="PRR"]But.... at this feature/price point, with the right promotion... this box could sell like hotcakes.[/quote]
I think this was the sort of idea they were after. They actually have sold a lot of these boxes!

[quote author="PRR"]SM Pro Audio does not seem to have a "high-price" line[/quote]
Depends on your definition of "high-price". They have the "step-up" PR8mk2 (USD$200) (with a slightly different topo and actually uses a pot for the 4.7k resistor! for gain setting) that has a couple more features, nicer cosmetics and better opamps, with the option to add an ADAT output for USD$100 more. These guys are pretty sharp IMO, going for the home recording market in this way. Their product line-up seems to be filling up with "good" ss/"better" ss/"best" tube (probably a starved plate) products, yet they are staying in the price point where nearly anyone doing recording can afford their products. It ain't an OctoPre or a TRUE Systems Precision 8...but its a tenth the price, at least!!!!

Great discussion and some great food for modification ideas! I'll get cracking and make some mods and measurements, hopefully this week.

[quote author="PRR"]"Our staff are dedicated audio professionals and design engineers who have a passion for product research, and are committed to bringing outstanding products to market."[/quote]
They probably copied that from Nady's website!!:green:

Peace!
Charlie

 

[PRR]

> Regarding the balanced output circuit - ...the circuit is a little difficult to model mentally, and I can't quite figure out what it will do, without looking for, and finding, my pencil.

Dog ate my pencil so I used a blunt hammer: SPICE.

Within the assumptions of perfect parts, and assuming the simulation does not get stuck or caught in a crack, SPICE will often give the right answers. In this case I used perfect resistors and a near-perfect op-amp with zero DC offset, infinite GBW, and gain of 10^6.

For example, with the 39K resistors omitted, it says the gain to the unloaded balanced output is "2", as we can see without a pencil. This is also the minimum gain you need if you want the output stage to clip before preceding stages (usually the best plan), because the push-pull output can swing double the swing of the single stages on the same supply rails.

With the 39K resistors, gain to the unloaded balanced output is 1.4300 (I eyeballed 1.2 or so). This means the stage before it will clip before the output stage (but see notes on output current).

> I'm sure that being 39k does actually screw something up

The outputs are not balanced. U1a (Ring) delivers 0.703V, U1b (Tip) delivers 0.726V.

(Note that this thing would be very unhappy with one output shorted and driven to high level. However you could use the plug-halfway-in trick to get an unbalanced output.)

Remember, I simulated with perfect 0.0000...% resistors and part-per-million op-amp gain error, but get a 3% unbalance. This is not a real-world problem, but seems inelegant. This is a topology flaw, not that 39K is "wrong".

What it does best is reduce gain. Assuming that gain could not be reduced in an earlier stage (for whatever dumb reason), the "logical" alternative is to make R7 and R8 larger. And different. And given 5% values, you might not get balance any better than this 39K cross-couple. And it uses equal values, always nice in production.

I computed unloaded gains, because that is now the norm, and anyway all the feedback is ahead of the build-out resistor so loading reduces level just like a 47 ohm (per side) resistor.

> I would have expected the 39k resistors to both be 10k

U1b (Tip) output falls to zero (SPICE says 4uV). U1a (Ring) gives 1.0000V or unity differential output. "Obvious" once pointed out.

> moving the cross coupling points (RH ends of the 39ks) to the other side of the output resistors (47R), with some kind of pre-load (say, 4k7) to ground.

That may have been what they thought they were doing. When done just right, you can show it unbalancing perfectly when one output is shorted. Me, I mostly do not want my outputs to be that clever because there are so many stupid loads in the world. If I need a true floating output, I use iron.

> even when using 1% types... you often need a preset somewhere to trim the output signals to be equal.

The price of 1% has fallen, but presets don't fit in $10 production budgets.

> steal useful current from the potential external load.

Which is a delicate subject using low price op-amp chips. I remember when 2K was minimum load, and even the "audio" chips rarely claim lower than 600 ohms. But in this bridge, IF you had a true 600 ohm load, each side sees 300 ohms. TL072 will run out of current (or gain) before clipping in 300 ohms. 5532 does better but is working beyond its specs (unless you use 300 ohm build-out resistors for true 600 ohm source impedance).

 

[AP]

His circuit is just totally broken...

As you say, a simulation shows both outputs providing a gain of around 0.7 each. ie. Two times 0.7V out for 1V in - an aggregate gain of around 1.4

It is easy to see that if you ground the output of U1b, then the gain through U1a is (-) 1

If you ground the output of U1a, then with a little pencil work, you can see the gain of U1b is around 0.88

So the gain changes according to the disposition of the load (hardly a worthy design aim).

To 'fix' this circuit, change the 39k resistors for 10k resistors, and *loose* R1 - it simply shouldn't be there.

For good measure, move the RH ends of R10 and R12 to the outer ends of R13 and R14...

*Now* -- ground the output of either U1a or U1b, and the gain to the outher output is either x 1, or x -1 (as appropriate...)

But leave both outputs open circuit, and the output of U1a will be very small (a few millivolts), and the gain to U1b will be just a tad short of 1.0, the net result being an aggregate gain of 1 (within normal limits).

-- the outputs are being unbalanced by the different effective impedences presented to the outputs by R12 and R10. Output U1b is seeing pretty much R3 and R10 in series (ie. 20k), but U1a is seeing R12+R8 (also 20k), but ending up on a signal about equal and opposite... so it looks like 10k..

This can be roughly sorted out by changing the output load resistor R6 for a 9k1 (it is close enough for jazz).. Lo & behold, each output is now producing abaout half the input signal, equal and opposite.

It doesn't really matter that it doesn't balance quite exactly, as long as the aggregate output is consistent (and it will be). Small differences in the external load will tend to unbalance it further anyway (the same thing would happen with a floating transformer balanced output too!)

Pros & cons of this type of circuit? Well, if the output *is* running balanced (and showing equal and opposite) you will have twice the headroom of an unbalanced circuit. Unbalancing it, by grounding one output looses that benefit though...

The real advantage (apart from the ovious benefits of balancing) is simply the convenience of being able to plug an unbalanced (TS) jack in, and it all works without stress (you can see that the output that looks 'shorted' is no longer actually driving, because the other output has doubled, and that doubled signal is being subracted from it's own signal, the net result being nothing to output).

Is it necessary? Not really. Simply having two equal and opposite signals as balanced outputs (remove the two cross-connecting resistors (R10 and R12). This gives you all the benefits of a balanced output, and behaves effectively just like an output transformer with a centre-tap (or a center-tap, if you like!). All you have lost is the convenience of easy unbalancing (as you would with a centre-tapped trafo -vs- a fully floating transformer output).

And I'm with PRR when he says "there are so many stupid loads in the world"...

Even the ubiquitous electronically balanced (differential) input using an op-amp and 4 resistors (the one that this balanced output is based upon) is a lot less than ideal in this respect. Putting this balanced output into one of those will unbalance it nicely, and you will see a significant common-mode signal on the wires... (but thats another story...)

Alan

 

[adrianh]

Ok group I am slow.

Did we conclude that the Nady and the PR8 were the exact same thing?

 

[SonsOfThunder]

[quote author="adrianh"]Ok group I am slow.[/quote]
Huh! Talk about feeling slow! Have you made sense out of PRR and AP's postings? I have an EE degree and its taken me a couple of run-throughs just to get some handle on things. I actually printed their last posts out so I can go over them with the schematic. This, of course, was a large part of my motivation for posting the schemo...well besides hopefully helping out some fellow DIYers who want to buy one of these to modify...was to learn more about the mechanics of this topology specifically, since this and similar things seem to be so popular. This one is more basic, so if we learn all about this one, then we will be more able to assess the variations thereof in a more complete way.

Did we conclude that the Nady and the PR8 were the exact same thing?

No such conclusion...
[quote author="SonsOfThunder"] Btw, I asked SM Pro if they were building these for Nady...and the reply... basically "Nady saw ours and ripped us off". So I'm not sure how close the Nady is to this. [/quote]
The externals certainly make it LOOK the same...I might have to buy one and open it up to see how close now!

Peace!
Charlie

 

[PRR]

> Did we conclude that the Nady and the PR8 were the exact same thing?

"Same on the store shelf": 8 inputs for $99.

And since solid-state matured, knobs+jacks/dollars has been an important commercial specification, even if we never actually say "Wow, just $7.84 per knob!".

As far as the Marketing side goes, the two boxes compete head-to-head.

We do not have internal details on the Nady, can't comment on internal similarity. But since there is no great virtue in the SM Pro design, and hints are that they don't cooperate with Nady (these days, you never know who owns, is-owned, or sub-contracts with who), it is probably not the same guts. (Unless both SM Pro and Nady bought the whole product from some anonymous Chinese designer and factory.)

> His circuit is just totally broken...

I would not say it that way.

The balance is more than good enough for most any use; 3% unbalance is nothing. And at $10, I would not be shocked to find a simple unbalanced output. The only reason you never see that at line level is that not everybody agrees which XLR pin is hot. (Though with 1/4" TRS, it seems clear.)

It looks like a cross-coupled auto-balance stage, but clearly is not. It may only be an odd but perfectly acceptable way to reduce output stage gain. (Why output gain needs to be reduced from its natural "2", after putting in two stages of gain, is a different issue.)

> So the gain changes according to the disposition of the load (hardly a worthy design aim).

Inelegant, and probably not a "design aim". It is rational to give unbalanced loads -6db or -12db gain relative to a balanced load (-6dB because bridge outputs make twice the supply voltage and simple unbalanced inputs may clip; -12dB because that is the difference between "Pro +4dBm" and "semi-pro -10" levels).

But in a box like this, you connect a load and trim the "gain" for nice output. The fact that the gain is different for a different load connection is hidden by the fact that a different input will also generally have different nominal input level. And by the probable markets for a 8/$99 box: low-tech users who plug, trim, and play, and don't know a dB from a KHz; or old farts like me who -can- level-match and compute impedances, but some days just want to plug, trim, record, without caring what the electric levels happen to be.

> you will see a significant common-mode signal on the wires...

But what is "significant"? I figure it is the receiver's (input's) job to reject common-mode voltages. If driving very long lines, the source can't force balance at the far end because of line resistance (though this would never be true in music studio work). In many cases, unbalanced outputs feeding good-CMRR inputs works just fine.

One objection to high common-mode signals on lines: they radiate into other lines. In small systems with shielded cable, this should not be a problem (if it seems to be, it is probably common-grounds destroying the differential-inputs' CMRR). In large systems it can be. And of course running unbalanced high-line-level in a snake with high-gain mike circuits is begging for trouble (yet I got away with it many times).

 

[AP]

You are very forgiving, I'll say that!

Alan

 

[dejacky]

Someone mentioned this earlier and this is EXACTLY my biggest problem with MY PR8:

The preamp cannot handle hot/loud signals and it distorts VERY easily. What would be a good mod to only compensate for this? or mod it so that the preamp has more headroom so it doesn't distort so easily when I record drums... (yes i already have the -10dB pad engaged on all mics..still not enough) :?:

 

[BYacey]

I hope I am not to forward in suggesting to bring negative feedback back to the emitters of the input transistors from the outputs of U1a/ U1b. I would also eliminate the level control and put in the anti-log pot between the emitters. This seems to make more sense overall. As PRR suggested, those 470pf caps look like they severely crunch down the top end bandwidth. Instead of "sparkle" you get "thud". I believe that Valley / Allison made on of the finest xfrmrless input preamps even by todays standards using the negative feedback to the emitters.

 

[dejacky]

I've tried understanding this thread to the best of my knowledge, but cannot figure out how to solve the following issue of my PR8:

The PR8 "clips" (distorts) hot mic signals because it cannot handle the high spl's coming from the mic. What modification would help prevent this?

Any help would be GREATLY appreciated .

 

[rafafredd]

Just strip out all the electronics inside and build some nice preamps in there. :green:

Just kidding, but that´s what I would do.

Maybe you should try a pad at the input.

 

[PRR]

The input stage won't overload up to 1.4V input. If you are getting 1.4V with a -10dB pad switched in, you should play softer.

The gain control should reduce that; don't be afraid to turn-down to 2 or even 1 (on a 0-10 scale).

Are you sure the mikes themselves are not clipping?

If you still have too much gain: you probably don't need an amplifier, just a way to get Phantom onto your mikes and adapted to a Line In.

But you can try this:

First, clip one end of R28. That gives you another 5dB headroom at the input.

Find R26, a 100K next to U2.B. Tack a 10K resistor across it. Gain will be 20dB lower. This will allow you to set the Gain higher, and keep the noise low.

If you are STILL clipping at the input: remove Q1 Q2 R17 R18. At each transistor location, put a 10K resistor from B to C. Now the input stage has gain of 0.8. Max input is around 10 Volts.

If you are sure the mikes are not clipping, a simple passive 20dB pad in front of the preamp may be the best bet, because ANY mikeamp is going to clip on hot-mikes on close-loud-percussion, so some 20dB pads should be part of your kit.

 

[SonsOfThunder]

dejacky i have used my modded PR8 to record drums (SM57s, Audix D6, C1000s, etc.) and had no problems with clipping. And it wasn't jazz...!

Here's the question I'd like to have the answer for...is the CLIP LED coming on, or are you hearing the clip in your track?

Rafa What would you put in, I wonder?

Bill which 470pF are you referring to? I already proved by measurement which caps needed to go bye-bye. Read thru the thread again. I like your suggestion to add neg FB to the emitters. I think changing the way the gain is set is a great mod that I will try soon.

HTH!
Charlie