Author Topic: Racking Gates "The Yard" Console Pre Amps  (Read 3474 times)

xfmr

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Racking Gates "The Yard" Console Pre Amps
« on: September 04, 2010, 01:20:15 AM »
I have a pair of these Gates "The Yard" Console Mic Preamp Cards that I would like to rack up. These were originally fixed gain with a trim following the output. I would like to change that obviously and make it into a standard, standalone circuit.

I already have a few things planned out but I had a few ideas to throw around so I welcome all input. My planned mods are:

1. -20db pad switch
2. Phase switch
3. Soft-start phantom power
4. MILA-style solid state power supply
5. XLR I/O
6. 1/4" Hi-Z Direct Input
7. Rewire output transformer for 600 ohm

Now for gain control I had a few ideas and I'd like to see what would be the best way to go about it. First off I'd like to replace R5 (220K fixed resistor) with a 250KA Pot. Then I'd like to make the feedback adjustable somehow. I was thinking of either a single switch, multi-position switch, or potentiometer in the feedback loop replacing R1 (56K) and possibly changing C4 (.47uf) with a larger value.

Ideas?



abbey road d enfer

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #1 on: September 04, 2010, 03:02:47 AM »
I have a pair of these Gates "The Yard" Console Mic Preamp Cards that I would like to rack up. These were originally fixed gain with a trim following the output. I would like to change that obviously and make it into a standard, standalone circuit.

I already have a few things planned out but I had a few ideas to throw around so I welcome all input. My planned mods are:

1. -20db pad switch
Easy. Just make sure its impedance is low enough to provide correct source impedance to the transformer and high enough to not unduly load the mic.
Quote
2. Phase switch
Easy too.
Quote
3. Soft-start phantom power
Why not? But why? Although it's undeniable that soft-start will prevent thumping on transformerless mic pres, it is not really justifiable when there's an input transformer.
Quote
4. MILA-style solid state power supply
What is a "MILA-style solid state power supply"? I've searched and googled it, to no avail...
Quote
5. XLR I/O
Seems to be almost mandatory, although I would suggest you add a 1/4" TRS to the output.
Quote
6. 1/4" Hi-Z Direct Input
Should be easy with a switching jack between secondary and grid. You will have to add a 1 Meg grid resistor.
Quote
7. Rewire output transformer for 600 ohm
That should be fine, will give you 6dB more output level.
Quote

Now for gain control I had a few ideas and I'd like to see what would be the best way to go about it. First off I'd like to replace R5 (220K fixed resistor) with a 250KA Pot.
An alternative is to install a pot directly in the grid of the first stage. That would make it bullet-proof. Would not be the best in terms of noise performance, but would permit some interesting grit. To consider if you expect very hot signals.
Quote
Then I'd like to make the feedback adjustable somehow. I was thinking of either a single switch, multi-position switch, or potentiometer in the feedback loop replacing R1 (56K) and possibly changing C4 (.47uf) with a larger value.
It looks like the circuit operates with about 25dB of NFB, so you can possibly increase the gain somewhat, at the cost of some increase in THD.
You can also decrease R1 in order to decrease gain, but that would not really be beneficial, because the increased load on the output stage will start do degrade the headroom. Stability may be an issue too.  33k seems to be the inferior limit to the exercise. I don't think you'd really need to increase C4. A brief sim doesn't seem to indicate it needs to be. All in all, a 250k Log pot with a 33k padding res should do the trick. Should provide -5/+12 adjustment.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

lassoharp

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #2 on: September 04, 2010, 03:31:57 AM »
I think emrr has more detailed experience with the specifics of these.  My general suggestions would be:

For the volume control, I would be asking whether I wished to try and maintain factory specs with regards to fidelity, distortion etc.  If so, using a 3 position 10, 15, 20db or finer resolution pad on front would be the cleanest way. If you wanted more of a nuke n smash amp it may not matter.

May be best not to add the pot without removing all or most of the FB in either case. First thing I'd do is remove the FB and measure the response ( and compare to response with FB ). That would help in deciding whether to ditch the FB entirely or ditch the 250K pot and control from FB value. (Gates did once use a pot inside a small loop)

I'm going to vote using a switch to change FB resistor value and cap value all in the name of finding what's best for your needs.  It's impossible to guess that but an answer will be there.  If you go larger than .47(on 56K) you're going in the direction of moving the caps influence on frequency response further away from the audio band. I do think you can get more overall gain via changing the FB resistor value and not suffer too severe changes in distortion and freq response.

As with any proposed change there's always the question - "What is the as-is circuit not doing that I wish it to do?"


emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #3 on: September 04, 2010, 09:50:37 AM »
I did that interstage volume control mod once, before I knew better.  You can't, since it's inside the feedback loop.   Feedback variation is your only option.    FB is high here, like 25 dB, and turning an interstage down actually decreases input headroom while decreasing output volume and increasing distortion. 

Grid pot in the first stage possible too, leads need to be short as possible.  Few inches total. 

Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #4 on: September 04, 2010, 03:12:31 PM »
Quote
What is a "MILA-style solid state power supply"? I've searched and googled it, to no avail...

Sorry, the MILA is a Mic Preamp designed by NYDave. I have attached the power supply schematic (I hope its OK to post it here).

Quote
For the volume control, I would be asking whether I wished to try and maintain factory specs with regards to fidelity, distortion etc.  If so, using a 3 position 10, 15, 20db or finer resolution pad on front would be the cleanest way. If you wanted more of a nuke n smash amp it may not matter.

Cool, I think a 3 position pad may be a great idea here. I want it to stay clean, but I did want to goose a little more gain out of it which is why I wanted to mess with the NFB. What would be a good set of values for the 10 and 5 db settings? I was going to use a pair of 1K with a 220R across it for the 20db, so IF that is right for -20db do I just halve and quarter the values for -10db and -5db settings? Can I keep the 220R or does that need to be tweaked on the other settings as well?

Quote
I did that interstage volume control mod once, before I knew better.  You can't, since it's inside the feedback loop. Feedback variation is your only option. FB is high here, like 25 dB, and turning an interstage down actually decreases input headroom while decreasing output volume and increasing distortion.

Aha, I had a feeling that would be the case, part of why I posted to begin with. Good to know.

Quote
Grid pot in the first stage possible too, leads need to be short as possible.  Few inches total.

I thought this was a big no-no. Wouldn't I be OK with just the multistage pad and the NFB control?

Quote
Seems to be almost mandatory, although I would suggest you add a 1/4" TRS to the output.

Yes, XLR AND 1/4" TRS on output, with XLR and 1/4" TS Hi-Z on input

Quote
"What is the as-is circuit not doing that I wish it to do?"

All I really want is access to a little more gain, adjustable gain, pad, phase, + phantom. Sounds like I have a good plan for all of that now.



lassoharp

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #5 on: September 04, 2010, 03:57:04 PM »
Cool. You'll probably be good to go with that plan.  You may get lucky if the amp doesn't go crazy out of whack with the FB totally removed.  It may be useful to have no FB or very close to it on your switch for a crunchier setting.

Here's a good recent thread on pads.  As abbey road points out, less than 20db is hard to make consistent and less than 10db is even harder. Good thing is that they will still work. Problems would be case to case. A rise in the high freq response is as good a candidate as any for a potential problem with a given pad network.  Something like that could be a welcome thing or not.

http://www.groupdiy.com/index.php?topic=40647.msg503589#msg503589

xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #6 on: September 04, 2010, 09:45:32 PM »
OK, now I am thinking about a 2-pole, 6 position switch for gain control. A fixed 33k resistor, then varying values starting at the 33k for the first setting and ending with a 'nuke' setting switching in a 50uf bypass cap (with a 47k resistor across the last switch to prevent pops), the bypass effectively erasing the feedback loop. Then I will go with a 3 way pad on a 2 pole, 4 way switch, -10db, -20db, and -30db settings. SO, my new questions are...

1. Will: 2x 470R with 220R across them, then add 2x 1k in series with 470R, and then 2x 1k5 in series with 470R and 1k work as roughly -10db, -20db, and -30db pads respectively?

I'm trying to figure out the right approximate values and how to actually wire the 4 way switch and this is what I came up with.

2. What values should I use for the 4 middle settings on the 6-way gain switch for measured gain steps?

I'm not even sure what max value to use here for setting 5, and I'd like setting 3 to be near the stock value. I'm thinking 33k for position 1, 10k in series with the 33k for position 2, then 22k in series with 33k and 10k for position 3, then 47k in series with 33k, 10k, and 22k for position 4, 100k in series with 33k, 10k, 22k, and 47k (with a 47k resistor going from the cathode to the + of a 50uf bypass cap) for position 5, and finally another 100k in series and a 50uf bypass cap switched in across a 47k resistor for position 6
« Last Edit: September 04, 2010, 10:26:59 PM by xfmr »

emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #7 on: September 04, 2010, 10:30:28 PM »
You are thinking way too much.  You need to wire this up on the bench, and get proof of practical performance. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #8 on: September 04, 2010, 11:15:02 PM »
Quote
You are thinking way too much.  You need to wire this up on the bench, and get proof of practical performance.  

Of course, I always overthink! I was just looking for a good place to start and looking for parts I need to order. Of course I'm going to check it out before I wire it up as a finished piece. Now I'm thinking I only need the -20db pad on a regular DPDT switch, but I still think the 6 position gain control switch could be cool. It gives me variable gain and the ability to remove the feedback completely while cathode bypassing the first stage. Are my guessed values no good?
« Last Edit: September 05, 2010, 02:00:49 AM by xfmr »

lassoharp

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #9 on: September 05, 2010, 01:06:07 AM »
Quote
Will: 2x 470R with 220R across them, then add 2x 1k in series with 470R, and then 2x 1k5 in series with 470R and 1k work as roughly -10db, -20db, and -30db pads respectively?

If you look in the pad thread from the above link you will see a formula that will give you the exact db attenuation value of these or any resistor values you choose.


xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #10 on: September 05, 2010, 02:06:45 AM »
Quote
If you look in the pad thread from the above link you will see a formula that will give you the exact db attenuation value of these or any resistor values you choose.

Right, that is where I got that info from. I think though that a normal -20db pad with two 1k resistors and a 220r across them will be all the pad I will need.

Now I just need to to figure out the best way to control NFB. I'm going to try out a 6 position, 2 pole switch unless anyone thinks a pot would be better. I'm thinking the advantage the rotary switch would have is being able to have a 'nuke' setting with a bypass cap wiping out the feedback on the most extreme position. I've done this type of circuit on guitar amp feedback circuits and really liked the jekyll and hyde nature of it. If I used a pot and a separate switch then when the switch is on the pot would be useless and I don't like that.

MagnetoSound

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #11 on: September 05, 2010, 05:38:07 AM »

If you use that PSU circuit, take care not to forget that the filament supply is floated at HT potential.
Dan

I don't think people realize what an embarrassment of riches this place is. - Paul Gold

xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #12 on: September 05, 2010, 11:30:44 AM »
Quote
If you use that PSU circuit, take care not to forget that the filament supply is floated at HT potential.

Right, I know it will reduce hum and that is the reasoning behind it but will that cause any problems for me? Can I still run a pilot light off the heater supply?

emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #13 on: September 05, 2010, 03:29:00 PM »
It's at about 30V, not 300V.   1M and 100K string from 329V.   300V would exceed heater/cathode specs. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #14 on: September 05, 2010, 03:34:19 PM »
C3 is small.  C4 is same in feedback.  Make C4 bigger, you may lose low end.  Make C3 1 mfd before making C4 larger.   There's likely a balancing act there. 

Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

abbey road d enfer

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #15 on: September 06, 2010, 12:05:46 PM »
Quote
You are thinking way too much.  You need to wire this up on the bench, and get proof of practical performance.  

Of course, I always overthink! I was just looking for a good place to start and looking for parts I need to order. Of course I'm going to check it out before I wire it up as a finished piece. Now I'm thinking I only need the -20db pad on a regular DPDT switch, but I still think the 6 position gain control switch could be cool. It gives me variable gain and the ability to remove the feedback completely while cathode bypassing the first stage. Are my guessed values no good?
A good balance between target-shooting and overthinking is simulation.
Unless you're on Mac, LTSpice is just the ticket. But the results will be just as good as the parameters you enter, so the results will be probably within 2-3dB. If you want real accurate, you need to adjust on the bench.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #16 on: October 10, 2010, 03:17:09 PM »
Some relevant bits concerning feedback changes in a pentode based preamp.

http://www.groupdiy.com/index.php?topic=41032.msg513354#msg513354
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

PRR

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #17 on: October 11, 2010, 09:04:25 PM »
> a pentode based preamp

Look where C2 is connected.

V1 is NOT working as a Pentode.

If we assume V2 has gain of 0.5 from grid to cathode, then V1 works as a triode with Mu of 21/0.5 or about 40.

This gives V1 gain near 30.

If V2 is loaded in 20K(?), it has gain near 12.

Open-loop gain 30*12= 360.

Closed-loop target (56K/1.2K)+1= 47.

Feedback margin 360/47= 7.5 or 17dB.

This will take-out 10+ dB sag, or take the 5%THD point down below 1%THD.

emrr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #18 on: October 11, 2010, 09:49:45 PM »
Yep, missed the C2 connection to V2 cathode.   Gates and their pesky multiple feedback methods.

I ran over here based on memory, and didn't have a close look again. 

As to the other thread, I'm confusing myself with the pentode issue/non-issue there......
« Last Edit: October 12, 2010, 01:07:57 AM by emrr »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

xfmr

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Re: Racking Gates "The Yard" Console Pre Amps
« Reply #19 on: November 15, 2010, 06:47:18 PM »
> a pentode based preamp

Look where C2 is connected.

V1 is NOT working as a Pentode.

If we assume V2 has gain of 0.5 from grid to cathode, then V1 works as a triode with Mu of 21/0.5 or about 40.

This gives V1 gain near 30.

If V2 is loaded in 20K(?), it has gain near 12.

Open-loop gain 30*12= 360.

Closed-loop target (56K/1.2K)+1= 47.

Feedback margin 360/47= 7.5 or 17dB.

This will take-out 10+ dB sag, or take the 5%THD point down below 1%THD.

Whoosh, most of that just went whizzing right over my head, but I will get there. PRR, how would you suggest I make the gain adjustable here by modifying this circuit?


 

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