Too much GAIN.

[bluebird]

So I took my new Philbrick preamp down to the studio and did what I always do, compare it to my 1272's and G9. I record drums and compare on different mics and different positions. I check out noise and high and lows.

One problem I always seem to have is too much gain. This is not a problem with the Neve 1272's or the G9. The 1272's don't have a pad and I've never had to use the ones on the G9. I know its not the input transformer because I'm using the same one in the G9 that I'm using in the other pres.

On both the langevin and the philbrick pres I overload the input not only with dynamics on kick and snare but also with a large diaphram condenser 8 feet away from the drum kit. Things are fine when I engage the pads but the noise floor goes way up.

on another post Dave (soundguy) said he uses pads on almost everything when tracking. I'm assuming he's talking about recording loud rock like me. So this might not be so out of the ordinary.

But why can the Neve take such a loud signal why the simpler circuits like the Langevin and the API similar Philbrick can't?

I have noticed on the API/Philbrick circuit there is a set resistor (20K) in the feedback loop. gain is manipulated with variable resistance into a large cap to ground. would manipulating this feedback resistance (20k) bring the gain down without messing with the frequency response?

By the way, heres the API schematic I used:
http://www.waltzingbear.com/Schematics/API/API_312.htm


Thoughts comments????

 

[Kev]

Look at your gain.
Not every Gain Block will have the same resistor values in the feedback to achieve the same gain. JLM99v and Fred99 and JH990 probably all have a subtle difference.
I'm tipping your Philbrick gain block is again different.

Check for a thread here with a post from Joe Malone about level through the trafo and then level through the gain block. I hope it was Joe.

Turns ratio of the input trafo can also have an influence on which breaks down first ... iron or op-amp. I'll take a wild guess and suggest the op-amp is crashing early.

Reduce the inherent gain in the circuit.
Have a juggle of R2, R3 and your pot.
No time to explain fully right now ... gotta fly ..... :roll:

 

[tk@halmi]

What is it turns ratio of the input transformer?
How did you wire up your rotary switch? Do you have a 20k/20k setting to get 2x (6dB) gain?
What voltage are you running the opamps on?
Do you have the spec for the Philbrick opamps?

If you reduced that feedback resistor you will get below x2 gain and it may
become unstable. That happens even more easily with crazy intruments like drums. Then again it may work.

Is your pad 20dB? You could make a 10dB or a 6dB pad instead and get less noise. How about that?

Tamas

 

[soundguy]

bluebird-

what is the gain control on your 1272? If it is wired like a standard 1073 gain switch, the first couple of clicks are attenuators, so you would be running with a pad...

With langevin am16's and API 312's w/ 2622's, they'd be more or less worthless if I didnt have a pad with the mics I like to use. Thats completely normal 'round here. But with a -20dB pad, I dont have any kind of issues with a jump in the noise floor at all...

I record mostly rock and roll, but I also record production sound for movies and have to use a pad quite often whenever actors are yelling, it really so much depends upon a)the mic you are using and b)the gain structure behind it. Seems to me though, if you know the transformer can handle the level you are throwing at it and you are positive about it, its pretty safe to assume that its the opamp. If you have the feedback right, then you need a pad. I think the smartest thing to do would be to figure out where the opamp is at unity gain and see what kind of results you can get there and then worry about how you are going to boost the gain of the opamp later on. Unfortunately thats about where my good advice comes crashing to a halt, I dont know how you'd go about determining where the opamp is sitting pretty unless you had the spec sheet for the opamp. Perhaps someone here could tell you how to determine where unity is. Thats something Id like to know myself.

dave

 

[bluebird]

Wow, thanks for the input!!

What is it turns ratio of the input transformer?

The turns ratio of the input transformer is 1:2.8

How did you wire up your rotary switch? Do you have a 20k/20k setting to get 2x (6dB) gain?

As far as the switch I just used 11 different values from 47 ohms to 6.8K

The feed back resistor is actually 22K like in the JLM99v
http://www.jlmaudio.com/JLM99XMB.pdf
I'm sure I have a 22K in the switch.

What voltage are you running the opamps on?

+/- 15V

Do you have the spec for the Philbrick opamps?

EDIT: specs down a couple...

I hope you can see this pic ok

 

[Kev]

[quote author="soundguy"] Perhaps someone here could tell you how to determine where unity is. [/quote]
yep
I was assuming there was something wrong. You do realy need to know what the correct set up for this op-amp is supposed to be ... rail volts and feedback arrangement etc. There shouldn't be too much to change.

Try an external 10dB XL style inline pad as an experiment.

 

[bluebird]

The 1272's I have were racked by Dan Alexander. I got it about 7 years ago. so maybe the first couple of notches are acting as pads....but the noise/hiss is very quiet comparing the signal to the Philbrick with the pad on.

I guess I should try to figure out unity gain with it there was a simple circuit I posted before:

 

[Mark Burnley]

Well,

One thing to remember with an op-amp in the non-inverting mode, is that you can't have a gain of unity unless it is wired as a "follower"- i.e. output fed directly to "inverting" input. The API-style circuits are all non-inverting setups but with gain settable via the "gain trim" pins.

If we go back to First Principles of op-amps, we can see how the resistor values effect the gain of the stage:

Gain= 1+ R1/R2

Where R1= the resistor in the feedback loop (the 20k shunted by 120pf)

R2= the "gain set" resistor plus any other impedance in the path from the "inverting input" node and ground.

So, you can see that the "1+" always features in the equation. So even with R1=20k and R2=20k, the gain will still be approxiamtely 2. The only way to get a "unity gain" stage is to leave the "gain set" resistor out-of-circuit. The R2 part of the term becomes huge (as good as infinity-ohms :wink: ), so R1/R2 becomes insignificant. The op-amp will then function as a voltage-follower. I can't read that spec sheet above- 15" monitor and early-morning eyes, so I can't tell you if it is "unity gain stable". That's one to try anyway!

Okay, next part of the gain mystery- the input transformer is 1:2.8, so that is a voltage gain of nearly 3. So assuming a gain of 2 at the op-amp, and 3 at the input transformer, that's a gain of 6 or 15dB-ish.

Just a few questions: What output transformer ratio do you have? Have you decoupled the power pins to the op-amps with say 22uF electrolytics and 0.1uF ceramic caps? Strap them from +Ve to 0V and from -Ve to 0V (watch the polarity!) I've had problems with a "distorting op-amp" before, only to place an AM radio nearby and notice some crazy sounds in the radio which magically >disappeared< when I switched off my box. Popping a few PSU decoupling caps near the op-amp shut the thing up. It had been oscillating a about 1MHz. This was undetectable on the 'scope, but came thru loud and clear on the radio! Because of this HF oscillation, any audio signal was saturating way too early, as a lot of gain and headroom was lost to this oscillation! :roll:

BTW, have you put R4 and C5 in the circuit as the transformer termination? It may not be correct for your transformer. I have a post by CJ with words from Jensen(??) somewhere on my HD about how to set up correct termination for a mic transformer. I'll see if I can dig it out. And have you got a better scan of the datasheet?

Have an experiment with pads and different gain settings, and let us know how it goes :thumb:

Great looking project BTW!

Mark

 

[peter purpose]

PHWOAR......I love Mark's explanations......I feel I almost understand them..!!

Ian.....On my API's, I always tie the low input to 0V (what would be pin 4 to pin 3 on a 325 card) and as I've 'always' done it, I don't know whether this would be the answer to your problem.

peter

 

[Bauman]

Blue,

I'm into rock record all the way and seems that you'll really need the pad at input of API or API style preamp. I do. You can raise the PSU to +/-20 if it's possible, it'll give you a little more headroom, but didn't proved if it solves the overload issues on my APIs yet.

I've never used those -10dB on my mikes, now I will.

cheers!
Fabio

 

[bluebird]

Ok I'm starting to understand. here are the specs a little bigger:
yes I do have 100uF right by the supply pins.

and here is the explaination of the trim pin since I have the PP65A and not the P65A: needing the external bias resistor:

on the side of the opamp it says OFFSET TRIM 20K and I+ TRIM 200M and I- TRIM 200M

so that means I should put a 20K resistor between the trim pin and V+? I have a 56K resistor there now because it worked...

Oh and mark I dont have the transformer termination cap and resistor on there. The output transformer is a 20ohm to 600ohm.

thanks so much.

 

[Kev]

[quote author="peter purpose"]PHWOAR......I love Mark's explanations......I feel I almost understand them..!![/quote]


yes ... thanks Mark,
as always.

 

[bluebird]

Ok so I've been messing with the 20K feed back resistor and this definitely lowers the gain. from about 700 ohms to 0 ohms gain stays constant. the variable resistance between that node and the 1000uF cap to ground does not make a difference when the feedback resistor is between 700 and 0 ohms.

So should I just have two gain controls? One varying the feedback resistor from say, 700 ohms to 20K, and another varying the resistance from that node to the cap going to ground?

Or just set one or the other to suit my needs...

 

[soundguy]

I used to put a control on everything and now Ive found that less is more usually. I have a lot of preamps, so its easy for me to say that this is good for this alone, but usually on the boxes that I put all kinds of pots for gain control on, I took them out later as often I found the amp worked best in an optimum range. Play around with it in your studio and see how you like it best and then fix a resistor to give you the right gain range with a single pot, thats what I would do.

dave

 

[bluebird]

Yes, I agree. I usually only use the first 3 gain settings on most of my pres. What attracted my to the whole opamp pre thing was the simplicity.

I too use to be a bells and whistles guy. But I've learned that all those switches and knobs end up only looking cool after a while.

:thumb:

 

[PRR]

Couple problems here.

The Philbrick app-note shows a gain of 1:101. With your input iron gain of 1:2.8, this gives almost 50dB gain, way too much for close-miking with hot mikes.

For today, rewire the feedback like this:

There are some potential problems here (pot-scratch will be deafening when the pot gets old; impedance isn't great) but it will let you try this thing through several sessions and get an idea of whether you want to keep working with this module.

With the 100K audio taper pot and 1K fixed resistor and 1:2.8 input tranny, gain adjusts from 280 (49dB) down to about 3 (9dB).

Another problem: this op-amp is rated 2.2mA peak output!!!! We call the TL071 weak because it is only 10mA, the 5534 will do well over 40mA.

If the rating is correct, this module will not drive 600 ohms. It will certainly be unhappy with many 600 ohm transformers even if the secondary is unloaded. The peak voltage in 600 ohms is 1.3Vpk, or +1.6dBm, 16dB lower than "any decent mike amp". And even before you approach 1.3Vpk it will probably distort and strain, because it is clearly intended for a 5K load. Open-loop gain in 600 will be a fraction of the spec-sheet value.

Wait, I just spotted this:

> The output transformer is a 20ohm to 600ohm

20 to 600??? That is another 1:5.5 of overall gain, or 64dB total gain (using Philbrick circuit). That's for far-miking choir, or some ribbon mikes, not close-miking with big capsules. Not to mention that the maximum output of the Philbrick in 20 ohms is 0.044 volts! Or less, if you omitted the output cap and have any DC offset at the output.

Reduce module gain as above. Bypass the output transformers: take an unbalanced signal from the Philbrick into a Line Input (check for DC first). See where that leaves you.

 

[Kev]

thanks PRR

looks like you have lots of gain and a very week output capability
perhaps this op-amp is best suited for gain or buffer sections and another op-amp for ouput stages.

 

[rafafredd]

Just my two cents...

You could try a simple buffer circuit after the opamp and before the transformer.

Or you could add a high turns ratio output transformer like 6:1 or 8:1 for lower gain and lower output impedance. So the low current output of the opamps will see a high impedance on the output transformer secondary.

But be aware that none of this things will help not to clip the inputs!

 

[bluebird]

Thanks PRR. Well that all kind of sucks.

Hows about something like this?

Edit: picture was to wide...


what about doing something like rafafredd suggested and flip the output transformer around?

 

[rafafredd]

I would use another transformer, like 10k/200 ohms.

If you insist to go with this transformer that you are using, you should add a buffer before it. even if you run it as 600/20ohms it will need a buffer.

what about this?:

http://hps.infolink.com.br/rafafredd/superbuffer_page1.GIF
http://hps.infolink.com.br/rafafredd/superbuffer_page2.GIF
http://hps.infolink.com.br/rafafredd/superbuffer_page3.GIF

(thanks sysmofit!)

Maybe you can translate it for me whyle you are at it...

But wait to hear what PRR says, as I never know exactly what I´m saying.