Author Topic: Preamp Design  (Read 6203 times)

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Offline JblissTopic starter

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Preamp Design
« on: January 21, 2018, 12:14:46 am »
Hi All,

I am looking at designing / building a Mic preamp attached is a schematic / design I'm am interested in. I understand the basics of this circuit and it operation. However it was be amazing if someone could explain, in further detail, it's operation in stages.
Including Information regarding CMRR and what to look for when trimming it ?
Any help would be greatly appreciated in furthering my understanding of preamp topology.

Thanks
Blissy
 

Offline RandallMcRee

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Re: Preamp Design
« Reply #1 on: January 21, 2018, 12:59:25 am »

Well its a Samuel Groner design. He is a nice guy and "hangs out" sometimes on diyaudio.com. You should post there.

It looks like a fairly standard low-noise design. There are a lot of specific component recommendations. Are you willing and able to follow them?

If yes, then this design is probably for you and you should post on diyaudio and get help there.

If no, you should go with a simpler design. Chances are good this is too much for you. It is definitely complicated for a first preamp.

Hope this helps,
Randy
 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #2 on: January 21, 2018, 01:17:08 am »
Hi RandallMcRee, Thanks for the reply I have already put together a parts list and sourced the majority of the components. I am up for the challenge, should be a good learning experience. I have tried to get into contact with Samuel unfortunately I was unable to. 
 

Offline RandallMcRee

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Re: Preamp Design
« Reply #3 on: January 21, 2018, 04:00:55 am »
OK, good first step.
Are you designing a pcb or wiring by hand?
CMRR trimming is pretty far down the road--you need to get the whole circuit working first. Do you have a scope and a signal generator?

The reason I ask about the construction method is that your overall quality (e.g. noise) will probably depend on your construction skills, e.g. single point grounds, low noise power and so on.

Are you going for low noise?:
Replace Q1-Q4 with 2SA970BL for very low noise

I would still recommend that you post at diyaudio, the folks here definitely tend more towards the EE spectrum and less the audio one. Samuel may not reply but there are many denizens happy to help. Myself, I have built pretty much everything except a mic preamp (I've looked at lots of designs....).
 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #4 on: January 21, 2018, 06:31:33 am »
Designing PCB in KICAD as we speak.  :D 
I have 4 channel scope and a basic Sig Gen 
have been doing a bunch of research regarding optimal grounding and low noise power. Taking this project slow using it as an opportunity to learn new skills.
 

Offline Richard Crowley

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Re: Preamp Design
« Reply #5 on: January 21, 2018, 06:53:47 am »
That circuit has been around for over a decade. I would have thought that there were PC boards (or layout files) available long ago.  Note particularly the notes in the schematic diagram and the cautions mentioned by @RandallMcRee
 

Offline Zero999

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Re: Preamp Design
« Reply #6 on: January 21, 2018, 10:36:24 am »
Anyone know how this works? There's some unusual things in there. What's going on with C10 and C11? Normally to make a non-polarised capacitor, two aluminium electrolytic capacitors are connected in series back-to-back, not parallel.
 
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Offline danadak

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Re: Preamp Design
« Reply #7 on: January 21, 2018, 11:23:35 am »
If you need a spectrum analyzer this one meets low cost criteria -


https://www.zeitnitz.eu/scope_en


http://www.zelscope.com/


http://www.ledametrix.com/oscope/


http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example   http://makezine.com/projects/sound-card-oscilloscope/



Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 
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Offline The Soulman

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Re: Preamp Design
« Reply #8 on: January 21, 2018, 12:02:22 pm »
Anyone know how this works? There's some unusual things in there. What's going on with C10 and C11? Normally to make a non-polarised capacitor, two aluminium electrolytic capacitors are connected in series back-to-back, not parallel.

C10 and C11 AC couple the gain switch, the bottom of the divider is 2 Ohms, in order to maintain that it is necessary to ad as little of esr
from the caps as possible (as this would alter the frequency response).
For any other application I wouldn't use anti parallel caps but here with very low voltage ac and minimum dc offset it is acceptable.

Would I build it? No. 
 
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Offline JblissTopic starter

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Re: Preamp Design
« Reply #9 on: January 21, 2018, 12:08:04 pm »
Anyone know how this works? There's some unusual things in there. What's going on with C10 and C11? Normally to make a non-polarised capacitor, two aluminium electrolytic capacitors are connected in series back-to-back, not parallel.

C10 and C11 AC couple the gain switch, the bottom of the divider is 2 Ohms, in order to maintain that it is necessary to ad as little of esr
from the caps as possible (as this would alter the frequency response).
For any other application I wouldn't use anti parallel caps but here with very low voltage ac and minimum dc offset it is acceptable.

Would I build it? No. 

You wouldn't recommend this design?
 

Offline Audioguru

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Re: Preamp Design
« Reply #10 on: January 21, 2018, 12:17:57 pm »
The circuit has way too many parts. It says its max gain is 66dB which is 2000 times that will pickup whispers a couple of blocks away. Why?
 

Offline The Soulman

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Re: Preamp Design
« Reply #11 on: January 21, 2018, 12:26:57 pm »
To be honest I don't have any experience with this design but to me it looks designed in simulation software with perfect components.
The real live implementation looks difficult and perhaps not very fault tolerant and giving the alternative's I wouldn't bother with this one, unless as a learning experience with regards to component and board parasitic's.

If the goal is to build a good working microphone amplifier I'd suggest something around a INA217 that has all the difficult stuff integrated into one IC.

http://www.ti.com/lit/ds/symlink/ina217.pdf
 

Offline dmills

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Re: Preamp Design
« Reply #12 on: January 21, 2018, 12:34:24 pm »
66dB is perfectly reasonable if you are potentially dealing with things like ribbon mics, you don't need it often, but there are times.

Most of the complexity is in the stuff around the outside, protection against P48 accidents, RFI measures, the usual.
 
It is a bit of an old school job and you can get similar performance out of a single chip these days (But you still need all that stuff around the edges to make a reliable design), the major upside of some of the modern parts is the use of a servo to do the DC control, makes for a much more reasonable cap value.

Have a look at the THAT Corp datasheets and app notes for a single chip design that works really rather well.

Regards, Dan.
 
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Offline Richard Crowley

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Re: Preamp Design
« Reply #13 on: January 21, 2018, 03:08:02 pm »
You wouldn't recommend this design?
Not clear exactly what your goals or experience (or desired result) are here?

That design is getting rather dated. It was state-of-the-art back in its day, but as others have mentioned, silicon technology and performance have improved since that day.  It is also one of the more complex, fiddly, and unstable designs you could have selected.

If you are looking for a challenge to occupy a few months of your spare time, it will probably fill the bill. But if you are looking for some good mic preamps for practical application in 2018, there are probably better/simpler solutions.
 

Offline ruairi

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Re: Preamp Design
« Reply #14 on: January 21, 2018, 09:52:24 pm »
If I was building a mic pre I would go where all of the mic pre builders hang out - www.groupDIY.com

Then I would decide whether I wanted something clean (SSL9K, THAT variants, GreenPre, Hardy M1 style) or something more colored (Neve ish or API ish for example).

Next I would pick a project that has a PCB available and lots of support and advice available from previous builders.

Then once you have completed that project and figured out power supplies etc you could consider doing a layout for your own project.  Per some of the above posters I could base that first PCB around one of the single chip solutions) - something similar to what Seventh Circle Audio do with their T15 - https://www.seventhcircleaudio.com/products/T15

I would suggest the T15 above as a great start for a clean pre and one of Jeff Steiger's Classic API mic pres for something more colored - http://www.capi-gear.com/catalog/

 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #15 on: January 22, 2018, 09:23:36 am »
Thanks everyone for there help! definitely considering all the options.


To be honest I don't have any experience with this design but to me it looks designed in simulation software with perfect components.
The real live implementation looks difficult and perhaps not very fault tolerant and giving the alternative's I wouldn't bother with this one, unless as a learning experience with regards to component and board parasitic's.

If the goal is to build a good working microphone amplifier I'd suggest something around a INA217 that has all the difficult stuff integrated into one IC.

http://www.ti.com/lit/ds/symlink/ina217.pdf

I am Interested in the INA217 however the output in the application circuit is unbalanced, however I assume I could add a Balanced Line driver circuit using a two opamps (ne5532) ?
 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #16 on: January 22, 2018, 09:46:33 am »
Or even easier I could use DRV134PA -  BALANCED LINE DRIVER possibly.
 

Offline IanMacdonald

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Re: Preamp Design
« Reply #17 on: January 22, 2018, 09:58:25 am »
The main point to keep noise (hiss) low is to avoid high value resistors in the input circuitry. If the resistor is shunted by a much lower value it doesn't matter (as with the 22k across a 600R mic) but if it is in series with the signal or 'floating' across it, it will induce noise. That also applies to the feedback loop, and is probably why a value as low as 2R was used.

All resistors in low level signal paths should be metal film, not carbon.

This comes as a surprise to most people, who think that transistors hiss.
« Last Edit: January 22, 2018, 10:02:56 am by IanMacdonald »
 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #18 on: January 26, 2018, 10:40:48 am »
Hi All,

Thanks for all your help so far. I have decided to start with an easier design based around the INA217.
Would be great if you could have a look over my circuit. Its based off the application notes and is pretty simple.
please let me know what you think and any edits. 

(excuse messy layout)

Thanks
 

Offline Richard Crowley

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Re: Preamp Design
« Reply #19 on: January 26, 2018, 03:05:44 pm »
Good choice (IMHO).  I believe that same circuit (and board layout) can also be used with That Corp. chips for the mic preamp and the balanced output.

The only two tricky things with that circuit are:
1) Matching the pair of 47uF NP capacitors.  I plan on using one of the little $10 component testers to select matched pairs of capacitors.
2) The low-value, reverse-log taper control for the gain. I have layed out a little round PC board that can mount on the back of the commodity 12-step rotary switch.  You can put 12 surface-mount (0805) resistors of your choice on the board.



 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #20 on: January 27, 2018, 02:06:12 am »
Good choice (IMHO).  I believe that same circuit (and board layout) can also be used with That Corp. chips for the mic preamp and the balanced output.

The only two tricky things with that circuit are:
1) Matching the pair of 47uF NP capacitors.  I plan on using one of the little $10 component testers to select matched pairs of capacitors.
2) The low-value, reverse-log taper control for the gain. I have layed out a little round PC board that can mount on the back of the commodity 12-step rotary switch.  You can put 12 surface-mount (0805) resistors of your choice on the board.






Thanks Richard thats perfect !

I am trying to work out the resistor values for a 12 step 1K series attenuator is there an easy way to do this?

Thanks
 

Offline JblissTopic starter

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Re: Preamp Design
« Reply #21 on: January 28, 2018, 03:30:09 am »
Hi All,

I this is my first PCB Layout it would be great if some one could let me know what they think.

Reagard the VCC and VEE nets currently I have them routed as traces daisy chaining round the board would it be more beneficial to route one as a power plane on the top layer ?


Thanks any help appreciated 
« Last Edit: January 28, 2018, 04:04:09 am by Jbliss »
 

Offline basinstreetdesign

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Re: Preamp Design
« Reply #22 on: January 28, 2018, 05:55:17 am »
JBliss:
Just for grins, here is a calculation of 12 resistor values for the switch described by Mr. Crowley.  You could have done all of the calculations by hand (and probably by now you have) but I did it using MathCad anyway.  I made a couple of assumptions:
1 - that you didn't need a 0 dB gain position for a mic. and
2 - that gain positions were 3 dB apart.  Let me know what gains you really want and I'll be happy to adjust the calculation.  Any monotonic mix of positive gains can be accommodated.
« Last Edit: January 28, 2018, 05:57:56 am by basinstreetdesign »
STAND BACK!  I'm going to try SCIENCE!
 
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Offline JblissTopic starter

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Re: Preamp Design
« Reply #23 on: January 28, 2018, 08:23:07 am »
JBliss:
Just for grins, here is a calculation of 12 resistor values for the switch described by Mr. Crowley.  You could have done all of the calculations by hand (and probably by now you have) but I did it using MathCad anyway.  I made a couple of assumptions:
1 - that you didn't need a 0 dB gain position for a mic. and
2 - that gain positions were 3 dB apart.  Let me know what gains you really want and I'll be happy to adjust the calculation.  Any monotonic mix of positive gains can be accommodated.

Thanks!!

The Values I roughly Came up with for an audio taper where
R1 1
R2 4.3
R3 9
R4 10.7
R5 14.4
R6 23.5
R7 36.8
R8 57.9
R9 93.5
R10 147
R11 232
R12 370
Total = 1000.1
(approx. depending on available Resistors)
Would these values work ?
 

Offline basinstreetdesign

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Re: Preamp Design
« Reply #24 on: January 28, 2018, 08:55:42 am »


Thanks!!

The Values I roughly Came up with for an audio taper where
R1 1
R2 4.3
R3 9
R4 10.7
R5 14.4
R6 23.5
R7 36.8
R8 57.9
R9 93.5
R10 147
R11 232
R12 370
Total = 1000.1
(approx. depending on available Resistors)
Would these values work ?

Well, theoretically they would produce gains up to 80 dB.  Are you sure you want that much gain in one stage?  Wiring the chip out to a 1 Ohm resistor for 80 dB gain will make an opportunity for the injection of noise into the signal via the wires.  I checked only the first 3 values (starting with 1) and found that they were not consistently spaced.  I think they were at 80dB, 75.5 dB, 57 dB ... or so.  Gotta get to bed now.  What Gains are you shooting for?  I'll take another look at it tomorrow.
STAND BACK!  I'm going to try SCIENCE!
 


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