[DESIGN] Original(?) mike-amp design

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PRR

Well-known member
Joined
Jan 30, 2010
Messages
11,143
Location
Maine USA
I've been pondering a somewhat different mike-amp.

There is absolutely nothing new in it. Yet I have never seen this exact plan before. (Citation of prior art welcome.) It has shortcomings, and the way I have fleshed it out it would be a poor commercial product. But it may be easy to build, be useful in many situations, look good, and generally be "an original design", as much as can be in this well-explored field.

Thoughts in my head:
  • Transformerless. I like transformers, but good ones are expensive. In effect you buy performance, and that isn't really DIY. (Unless you do considerable set-up to wind-your-own and learn all the little secrets, which is maybe too DIY.)
  • Low-Noise, of course. And while many designs give low noise at maximum gain, often the input noise level rises at lower gains. And with today's hotter mikes and sources, we often work at lower gain, but with today's 16-bit and 24-bit recorders, we often hope for huge signal to noise ratio even on loud sources.
  • "Professional" interfaces: differential input, differential 600-ohm output, +4dBm nominal with ample headroom. I still argue that "semi-Pro" interfacing is more appropriate for small studios and cleaner sound, but there is a historical legacy that is hard to fight.
  • All modern mike preamps have to have a +48V supply for Phantom. It seems silly to also have various other voltages like +/-15V. You can build a fine preamp with just +48V. The preamp will take more power than just the Phantom needs, but it is easier to beef-up the one supply than to build several supplies. Single-supply tends to force coupling capacitors, but Phantom already forces the use of caps. (Or transformers, but see above; or very clever common-mode design.)
  • S-i-m-p-l-e-! Don't make the audio go through a lot of tricky-stuff. Don't design something you can't easily build or debug or tweak.
  • Wide bandwidth and low distortion, but not obsessively so. A few-dB droop can be fixed in the mix. If the signal path is simple, the distortion will be inoffensive.
  • Feedback, because I am used to that type of design, and because it greatly simplifies setting of gain and balance.
Start simple. Don't worry yet about differential in/out, because there is a differential form of most single-side amplifiers.

First look at noise. All feedback amps can be represented in this form:

janus-2.gif


To change gain, you can change either feedback resistor. But for a number of reasons, changing Rshunt is most common in mike preamps.

Many mike-amps use 50K feedback resistance (Rseries), though for differential they may do it as two 22K resistors.

Mike-amp gain usually varies from 60dB down to 20dB, or even 0dB (unity gain) for hot mikes on loud sources. The diagram shows the other feedback resistor's value for various gains.

At 60dB gain, the amplifier sees 200 ohms of mike resistance noise at one input, and 50 ohms of feedback resistor noise at the other input. (Really 50 in parallel with 50K, but that's essentually 50 ohms.) The mike sees a total noise resistance of 250 ohms, of which 200 ohms is "good" (microphone signal) and 50 ohms is "bad" (feedback network dead resistance). Noise figure is 1dB, very good. If the mike maker claims mike self-noise equal to 20 dB SPL, we will observe noise equal to 21 dB SPL, which is about as good as we can get.

At 40dB gain, we have 700 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 5dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 25 dB SPL, which is not so good, though in many situations it isn't a problem. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 40dB gain for sounds of 108 dB SPL. In pauses, the noise floor is 25 dB SPL. The total dynamic range is 83dB, acceptable only because acoustic background noise may be around 25 dB SPL in many studios. (But I'm booked for a 108dBSPL gig where the noise drops below 15 dB SPL after rush-hour.)

At 20dB gain, we have 5,200 ohms of noise with only 200 ohms contributing signal. Noise figure has gone up to 14dB. If the mike maker claims 20 dB SPL, we will observe noise equal to 34 dB SPL, which is pretty poor. If we use a hot dynamic mike, and nominal preamp output level is -10dBV, we would use 20dB gain for sounds of 128 dB SPL. In pauses, the noise floor is 34 dB SPL. The total dynamic range is 94dB, barely 16-bit quality.

We would like to reduce feedback network impedance. But if we reduce Rseries, it loads the amplifier output, and increases the strain on the amplifier. Maximum output falls (or the output must be beefed-up), distortion rises.

Next: how many stages? A single-stage BJT amp "can" make 60dB of voltage gain, but not much more. When you also consider input and output impedances, it doesn't work. A 3-stage BJT amp can have a voltage gain of a million, but three stages is hard to apply feedback to, and simpler is probably better for sound. Let's start with a 2-stage design, though we may end up with one "stage" being a 2-device compound to get enough current gain to meet our impedance needs.

The most obvious 2-stage amp is sketched on the left:

janus-1.gif


Even you RCA B3C tube-amp fans will recognize this layout. Feedback from output plate to input cathode. The pins have new names on BJTs, but it has the same drawback. The 2nd stage plate resistor pulls up, but the feedback resistor loads it the other way. In low-gain tube amps the feedback resistor may equal the output load, doubling the amplifier's work and stress.

The amp on the right uses complementary devices to get the load resistor going toward the feedback network. In fact it works well if the load resistor IS the feedback resistor. And that gives the lowest feedback network inpedance, lowest noise for any gain setting, highest first-stage gain when needed.

I have omitted DC Bias, coupling caps, and differential operation. All these present problems, but it turns out that the solutions fit together elegantly.

---- enough for now. More later.....
 
Great plan.
I like the 48V-for-all idea - simple & obvious (with hindsight), but I never saw this before or thought of it.
:thumb:
 
It all seems simple enough. The words could describe much of any design we have seen before.
Keep it simple and add level.
see , easy isn't it ?
:green:
Often it is the components available that might set things up in a certain way and then the compromises come.

I've had similar thoughts that went along the line of a basic Tube circuit that is well proven and then substitute FETs for the tubes. This would mean a lowering of B+ ..... I had always thought to use a B+ that suited the FET but it never occured to me to set the B+ to 48 volts because it was convenient and simply made sense.

Perhaps it is possible to hit all targets at the same time.
However my thoughts had still included an input and output transformer.

It will be interesting to see which way PRR turns next.
Please continue - you have my attention. :wink:
 
I have a preamp that runs on 52V. It's discrete, single-ended and transformerless. Simple, 4 transistors. Actually, it's pretty damn close to that two-stage PRR thingy..

Input is unbalanced, because I'm using it with my Calrec mics. They're unbalanced and need 52V, not through that silly 6k8 deal though :roll:

I'm finishing up a version that'll run on 4x 12V batteries for field recording.

Not an original design. Front end is Rod Elliots pre:

http://sound.westhost.com/project13.htm

Output stage is something from an ol' Soundcraft mixer. I have a PDF..

I've done a simple FET pre as well, two JFETs and one MOSFET. Very small, nice mellow sound (I suspect some hf roll off in the design. Nothing that bothers me though, I'm anti linearity :green: )
 
I have other transistor stuff that runs at 48V or higher. I've experiemented with going as high as 300V. There is a change in sound for sure. And headroom :wink:

Audio Note once made a direct transistor copy of their famous M7 tube preamp (not for microphones - hifi stuff) where they replaced the tubes with MOSFETs but kept the high B+ PSU. I'm sure they adapted the circuit to work, but it sounded pretty darn good I think.

I always wondered why 24V seemed to rule. Another Neve wannabe thingy?

Volts are cheap! :green:
 
Unless you do considerable set-up to wind-your-own and learn all the little secrets, which is maybe too DIY
Tranzformerz rulez!
:grin: :grin: :razz: :razz:
Just my two cents:
some usefull solutions here:
http://www.geocities.com/igor_jazz/sch.html
see sch_collect.zip
(include very hot input mosfet stage of PA running ~ 0.5 A@+/-50v!!!!!

This topology was tried by me and my friend in Russia a lot of time ago
as basic for our micpre.... transformerless, +/-60 V @ 0.5 A,
we ended with designing of very nice PS and mind that we really don't
need all this, 'cause M**y can do it with 2 tubes and input trafo :))))
 
>I always wondered why 24V seemed to rule. Another Neve wannabe thingy?

Assuming you can squeeze 23Vpp output from that 24V supply voltage, that's about a +24dBu output.

...which is pretty plenty, enough headroom to drive the next equipment... whatever that may be.

Also, if you're using output transformers, as you go up above +18dBu, or +24dBu rated output trafos, they become more and more expensive.
 
I have a 1/2 built opamp 2406 preamp using 48 V. I did that for the phantom and to keep the electros polarized.

I also have another preamp started using a circuit something like the one PRR posted on TT that used two transistors.

I am interested with what this new preamp is
 
[quote author="owel"]>I always wondered why 24V seemed to rule. Another Neve wannabe thingy?

I think a lot of the 24V DC comes from Audio gear that was meant to run on emergency (battery) supplies when AC was unavailable. I have seen many industrial power amps that have back panel access for the 24V rail.

PRR, in your design won't output loading have an effect on the feedback? Would you require a further buffer stage to isolate the effects of varying output loads?
 
[quote author="PRR"]A 3-stage BJT amp can have a voltage gain of a million, but three stages is hard to apply feedback to...[/quote]

Well, there's always the classic "ring of three" topology. The main difficulty with this is getting all the DC levels correct for direct-coupling. You can "cheat" and AC-couple stages 1 and 2, and still have only one pole in the feedback network. Here's an example in vacuum tube form, reproduced from a book by John Linsley-Hood:
http://electronicdave.myhosting.net/miscimages/ringofthree2.gif

The gain is approximately (R7+R8)/R8. High-gain devices like bipolars or high-mu tubes such as the 12AX7 will remove most of the "approximately" from that statement for any realistic level of gain.

The R7/R8 ratio could be fixed by the lowest gain desired, and R8 shunted by a cap and switchable/variable resistors to give the needed gain up to the design maximum. This is conceptually equivalent to the gain switching in your first schematic, although done at AC only as to not upset the DC conditions of the stage.
 
With direct-coupling, each succeeding stages need a higher voltage than the previous section. But since we're working from +48VDC, we may have enough juice for the whole circuit.

P.S. The Yamaha PM1000 operates at +44V, direct coupled... but adjusting the 22ohm resistor to a little higher value and we can operate this at +48V. <hint hint!>
 
> With direct-coupling, each succeeding stages need a higher voltage than the previous section.

True if we only have one polarity of device which always has output higher than input. That is true of vacuum tubes; not for transistors which are available in both polarities. That is THE key reason transistors revolutionized direct-coupled design. Also it is possible to run transistors with collector voltages lower than base voltage when doing low levels. Three grounded-emitter NPNs in direct-coupled cascade can work with collector voltages of 0.6V, 0.6V, and 24V on the last stage. It would be a decent high-gain amp if not for the three R-C phase-shifts making it oscillate.
 
The 48V power is nice. when I did my Langevins (48V PSU) it was very easy to do phantom. If Langevin did it why can't we?...errr you I mean.

I like the idea of no output transformer. one of the best pres I've heard was a Lundahl input transformer into a millenia 990 with a big cap to output.

but an input transformer might be nice.
 
I'm disappointed that this thread seems to have met the usual fate of all "new design" threads here: buried beneath pages of clone posts.

Despite my past misgivings, you can now count me solidly in support of a "design/theory" forum.

How about these names for the forums?

Clone Lab
Design Lab
Brewery
 
This could be done with the subminiature tubes like the DI I did with a 6088. It is a tetrode that draws less than a miliamp and I successfully used a DC-DC converter for heater (direct heated) after following several peoples' advice. There are a few other candidates in this subminiature catagory that would work. They are available and inexpensive.
 
NewYorkDave and others, You have to remember that once the design reaches a "finished" state then it will immediately become subject to cloning... We seem to try hard to distinguish ourselves from run-of-the-mill cloners but I'm pretty sure each one of us started by building something from plans in a book, or online... Does that make us "cloners" because we did it before?

I'm in favor of the new section for designs but once they become more than just ideas thrown around, the "cloners" will come to the "design" section of the forums and quickly make it into another "the lab" with requests for PCBs and schematics to build to without pitching in.

this leads to other questions and thoughts like, who and/or what is a cloner? Do we setup rules to keep them from "cloning" and end up like RO with crazy policy where people get banned for "cloning" without giving something useful to the design? I for one am just getting the hang of designing simple Preamps and the like even though i've been doing power design for a while(actually fairly similar once you understand what you are looking at..) I came into this much after the GSSL and G1176 were "cloned" from the originals and I'm cloning them now because I want something that is superior for less money than I want to give to GC.. I threw a few layout ideas and suggestions out for the Forssell Comp but couldn't spare the time to proto and test the schematic, does that make me unfit to build it when PCBs finally get made?

sorry, I don't mean to poke at anyone's ideas I'm just thinking out loud here since if i actually do it out loud the woman will smack me.

:green:
 
I'm thinking that the idea of a separate forum is not to prevent others from cloning new ideas, but to keep the design ideas from getting swamped in the other posts. This way the flow of ideas will go a bit more smoothly if you don't have to spend time looking for the post. Then again if you post an empty message in a topic, you'll get an email telling you when there is a new post in that particular topic with an automatic link to it.

I think its a great idea, though i'm sure it won't work out exactly like we all expect it to.

Peace!
Charlie
 
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