[shot]

I'm in the process of calibrating my dual unit.
It's been slow since I don't have a lot of spare time.

What I noticed is that I have a lot of noise, both hum on 50hz and pink noise when going straight to output transformer without any series load. My output transformers are 10k:10k edcor. If I strap 100R to primary then the noise drops. I tried lower values like 22r and 47r but they don't work in my case. Also I need to figure out what is the best way to ground the unit. Usually I go for pin 1 to chassis but it seems that going unbalanced in likes better if I ground the pcb from pin 1 and remove pin1 to chassis link.
But I'm still reluctant if I should stay unbalanced or if I should get some input transformers...
And I'm thinking I should try 600:600 transformer for output... but that's a drag since I already mounted 10k edcors in chassis and wired them.

Soundwise, this compressor is rough. Sounds fantastic!



Luka

[Rob Flinn]

I'm in the process of calibrating my dual unit.
It's been slow since I don't have a lot of spare time.

What I noticed is that I have a lot of noise, both hum on 50hz and pink noise when going straight to output transformer without any series load. My output transformers are 10k:10k edcor. If I strap 100R to primary then the noise drops. I tried lower values like 22r and 47r but they don't work in my case. Also I need to figure out what is the best way to ground the unit. Usually I go for pin 1 to chassis but it seems that going unbalanced in likes better if I ground the pcb from pin 1 and remove pin1 to chassis link.
But I'm still reluctant if I should stay unbalanced or if I should get some input transformers...
And I'm thinking I should try 600:600 transformer for output... but that's a drag since I already mounted 10k edcors in chassis and wired them.

Soundwise, this compressor is rough. Sounds fantastic!



Luka

You would more normally be using a 10k:10k on input and something like a 600:600 on an output.  Hi  Z in & Low Z out

[fripholm]

What I noticed is that I have a lot of noise, both hum on 50hz and pink noise when going straight to output transformer without any series load. My output transformers are 10k:10k edcor. If I strap 100R to primary then the noise drops. I tried lower values like 22r and 47r but they don't work in my case.

Are you sure it's pink noise you're getting, not white?

The (optional) series resistor has nothing to do with noise, it's there to prevent the output stage from oscillating (when this happens, it sounds like tuning an old radio). Your 100R might be large enough for dropping the audio signal as well as the noise, so you won't improve anything there.

The circuit itself is very noisy by today's standards. It's much noisier than my 1176 for example for similar in / out and compression levels, but it's still workable. From my simulations most of the noise comes from the inherent high impedance of the Zener pairs, so there's no way to lower it - but this noise is WHITE, not pink!

You shouldn't be getting 50Hz hum, however. Try shorting the input and see if it goes away. Going un-balanced into the unit can pick up noise like this but you may also have a ground loop somewhere. My units do not hum at all.

These are the noise plots of both the 1176 (purple) and one of my Zeners (white). Input signal on both is an 800Hz sine wave at 0 dBu with 4dB of compression, output levels adjusted to be the same going back into the audio interface. The spikes at 50, 100 and 150 Hz are from the 1176.

[orangechili]

This is becoming a great resource! I notice a little more white noise in one channel than the other but its within an acceptable range.

I do notice a tiny difference between comp and lim modes between the channels. In one of the limit modes it doesn't compress as hard as the other channel. Fripholm, do you know what the AOT resistors for comp and limit would be located on your boards? (AOT 3 &4, 5 & 6 on the other build here)

I also notice a TON of compression even with the 4:1 step down transformer so tweaking those AOT resistors or adding the input pad may help me there. THD mode is way louder than comp and limit modes. I'm already at 20db compression on my meter with the input control up halfway. I know this thing likes to squash things to oblivion, I just want to see if there's a way to control it a little bit more.

It sounds amazing btw!

[shot]

Are you sure it's pink noise you're getting, not white?

It's white! Sorry!

The (optional) series resistor has nothing to do with noise, it's there to prevent the output stage from oscillating (when this happens, it sounds like tuning an old radio). Your 100R might be large enough for dropping the audio signal as well as the noise, so you won't improve anything there.

I have probed signal from various points (before resistor, before transformer, after the transformer) and it's definitely not dropping any level with 100r. I'll try to lower the value a bit. I went from 47r to 100r - haven't tried some values in between!

The circuit itself is very noisy by today's standards. It's much noisier than my 1176 for example for similar in / out and compression levels, but it's still workable. From my simulations most of the noise comes from the inherent high impedance of the Zener pairs, so there's no way to lower it - but this noise is WHITE, not pink!

You shouldn't be getting 50Hz hum, however. Try shorting the input and see if it goes away. Going un-balanced into the unit can pick up noise like this but you may also have a ground loop somewhere. My units do not hum at all.

I need to work on it a bit more. I did suspect it's a ground loop. Just had no time to work on it more.

These are the noise plots of both the 1176 (purple) and one of my Zeners (white). Input signal on both is an 800Hz sine wave at 0 dBu with 4dB of compression, output levels adjusted to be the same going back into the audio interface. The spikes at 50, 100 and 150 Hz are from the 1176.

Thanks for those plots!
I had my 50hz at around -80dbFS when measured in computer. But I'm sure I'll fix that.
White noise was at about -95 to -100dbFS (I don't remeber exactly)

I'm out of town until tomorrow so I have to wait a bit to continue debugging this thing.
But it's worth it! I've already built couple 1176's, pye comp, prr176, la-4 and clx160vu and this comp is fine complement in my arsenal!

Luka

[fripholm]

I do notice a tiny difference between comp and lim modes between the channels. In one of the limit modes it doesn't compress as hard as the other channel. Fripholm, do you know what the AOT resistors for comp and limit would be located on your boards? (AOT 3 &4, 5 & 6 on the other build here)

I also notice a TON of compression even with the 4:1 step down transformer so tweaking those AOT resistors or adding the input pad may help me there. THD mode is way louder than comp and limit modes. I'm already at 20db compression on my meter with the input control up halfway. I know this thing likes to squash things to oblivion, I just want to see if there's a way to control it a little bit more.

AOT3 is R40 on my boards and AOT5 matches R39. According to Fester's descriptions, AOT4 and 6 were not used in the TG1, so I omitted them.

I've already taken measures to raise the threshold a bit, but you're right; it still is a lot of compression. R56 (for THD adjust) is also part of this, as it somehow adjusts the output level. Remember, this is a feedback compressor and more level means more compression. You have to find some kind of middle way (don't know if this is the right term in English) between distortion, noise and output level. I didn't have to mess with AOT3 or 5 on my stereo unit. It tracked pretty well from the beginning - maybe it was just luck, I don't know...

The output level of your audio interface into the Zener is also important; maybe it's set very high? Mine is set to +13dBu and with the input control up halfway I have about 12~16dB of compression with a 2:1 input iron.

THD mode basically shorts the input to the sidechain and no compression occurs anymore - what had been tucked down by 20dB with compression now becomes 20dB louder - and distorted. You just have to keep this in mind when switching THD mode in... 

[brewbacca]

Hey guys, I'm having an issue where my V+ stays at 28V for about 2-3 seconds, and then drops significantly. If I disconnect R75, then my V+ stays constant at 28V.  Could this be related to current? Or could this have something to do with faulty transistors at Q13/Q12?

Thanks!

[fripholm]

You may have confused the PNP (Q12) and NPN (Q13) transistors? Check their orientation as well...

[mrscary]

Does anyone have a mouser cart for this one?

[Potato Cakes]

https://www.mouser.com/ProjectManager/ProjectDetail.aspx?State=EDIT&ProjectGUID=43808877-780f-4c00-b8a5-f1825d02c11d

I would double check it and also make sure that nothing is now on back order. This is what I used initially and I'm pretty sure I didn't forget anything.

I do not believe I added parts for the hold control option.

Thanks!

Paul

[innercityman]

https://www.mouser.com/ProjectManager/ProjectDetail.aspx?State=EDIT&ProjectGUID=43808877-780f-4c00-b8a5-f1825d02c11d

I would double check it and also make sure that nothing is now on back order. This is what I used initially and I'm pretty sure I didn't forget anything.

Big thanks for that !

[mrscary]

https://www.mouser.com/ProjectManager/ProjectDetail.aspx?State=EDIT&ProjectGUID=43808877-780f-4c00-b8a5-f1825d02c11d

I would double check it and also make sure that nothing is now on back order. This is what I used initially and I'm pretty sure I didn't forget anything.

I do not believe I added parts for the hold control option.

Thanks!

Paul

Thanks so much! Wow this makes things so much easier

This is for 2 boards, right?

[Ilya]

I'm in the process of calibrating my dual unit.
It's been slow since I don't have a lot of spare time.

What I noticed is that I have a lot of noise, both hum on 50hz and pink noise when going straight to output transformer without any series load. My output transformers are 10k:10k edcor. If I strap 100R to primary then the noise drops. I tried lower values like 22r and 47r but they don't work in my case. Also I need to figure out what is the best way to ground the unit. Usually I go for pin 1 to chassis but it seems that going unbalanced in likes better if I ground the pcb from pin 1 and remove pin1 to chassis link.
But I'm still reluctant if I should stay unbalanced or if I should get some input transformers...
And I'm thinking I should try 600:600 transformer for output... but that's a drag since I already mounted 10k edcors in chassis and wired them.

Soundwise, this compressor is rough. Sounds fantastic!



Luka

You might have noisy zeners. I've got the same issue and the difference in the noise floor between different zeners can be as much as 20 dB.

I describe the issue here as well:
https://groupdiy.com/index.php?topic=37400.msg839788#msg839788

[orangechili]

Ok I'm back, fun little project!

-I think a 10k log pot seems more natural on the input control instead of the 10k linear, compression curve seems better to me like this
-I tried the original input pad on one channel and like the gain structure on the original board design better. Would be cool to make it switchable I suppose...

And questions
1. I notice the with the input and output controls all the way down the signal does not get reduced to zero. I still hear signal coming thru. Is there a way to remedy this? Maybe experiment with R93(120ohm) for the input and R94(1k) for the output?
2.What components on the board should I modify to use these meters? They look similar enough to the Sifam ones for me. Here are the specs on the  meter
http://www.ebay.com/itm/TR-57-Panel-VU-Meter-Level-DB-Meter-Audio-Mixer-Tube-Power-Meter-w-Backlight-/192017460667


I am still fighting some slight excess noise but plan on tidying up some more connections with shielded wire, etc.

It's coming along nice!

[shot]

You might have noisy zeners. I've got the same issue and the difference in the noise floor between different zeners can be as much as 20 dB.

I describe the issue here as well:
https://groupdiy.com/index.php?topic=37400.msg839788#msg839788

Thanks for that Ilya!
It crossed my mind that it could be that when I first read about your issues with zeners.
Unfortunately I didn't have time to work more on my build. It's been busy few days.
I'll see if I can get different zeners and try them.

1. I notice the with the input and output controls all the way down the signal does not get reduced to zero. I still hear signal coming thru. Is there a way to remedy this? Maybe experiment with R93(120ohm) for the input and R94(1k) for the output?

I also noticed that. But I don't mind! If I'm going to use it in the mix it will surely be at some decent level so I don't care what level it is when it is completely CCW on ins an outs. But if you want I guess that you can lower those values or maybe omit those resistors and have it completely grounded and silent. Let's see what Fripholm has to say on this.



Luka

[fripholm]

1. I notice the with the input and output controls all the way down the signal does not get reduced to zero. I still hear signal coming thru. Is there a way to remedy this? Maybe experiment with R93(120ohm) for the input and R94(1k) for the output?

Of course you're free to just jumper these resistors to ground if you want zero compression or zero output. Personally I really can't imagine a situation where one would need zero output from a compressor. The input control adjusts for the amount of compression (obviously no compression occurs with it all the way down to zero) and the output control is the makeup gain to get about the same level out that went in - zero wouldn't make sense here as well 

IMHO the resistors provide a good range of control, but I realize that there are a million workflows and it's easy enough to modify things. 

2.What components on the board should I modify to use these meters? They look similar enough to the Sifam ones for me. Here are the specs on the  meter
http://www.ebay.com/itm/TR-57-Panel-VU-Meter-Level-DB-Meter-Audio-Mixer-Tube-Power-Meter-w-Backlight-/192017460667

It says 150uA for full deflection, which is much less than the meters it was designed for, so I would probably try and add a series resistor to lower the current.

[Potato Cakes]

Hello.

The last piece of the puzzle I need to build my stereo version of this is transformers. I'm wanting to use Sowters, and after discussing appropriate transformers for this type of project, they recommended the 9970 for the input and 9980 on the output. I was told they could add a lead so the 9970 could be wired 1:3.5 (backwards for input). I'm trying to make this build really special, so I don't mind spending a bit more for the cost of iron for this project. I just want to make sure these transformers would be suitable for this build.

Thanks!

Paul

[fripholm]

I'm wanting to use Sowters, and after discussing appropriate transformers for this type of project, they recommended the 9970 for the input and 9980 on the output. I was told they could add a lead so the 9970 could be wired 1:3.5 (backwards for input). I'm trying to make this build really special, so I don't mind spending a bit more for the cost of iron for this project. I just want to make sure these transformers would be suitable for this build.

I think a 1:3.5 (wired backwards) for input might be suitable. You just have to fiddle with R86 and R87, which control the bypassed level for unity gain - as discussed earlier in the thread.
For the output iron however, I don't see any benefit from using a high turns ratio. The output stage is perfectly capable of driving 600 ohms directly up to 25 Volts p-p (~21 dBu), so a step up is not needed if you just want to drive an ordinary line input. A step down of 7:1 would give you less than 5 dBu of maximum output level. A 1:1 or thereabouts would be more than appropriate. Sowter also offers a replacement for the TG12345 output amp transformer (Sowter 1365, turns ratio 1:1.77) which is the same price as the 9980/70 and might be better suited than those, IMHO. They also have a TG input transformer (1295, 1:3.16), which might be interesting when wired backwards.

HTH

[Potato Cakes]

Thanks for the response. I was a little confused with their initial recommendations for what I was asking. The 1365 and 1295 seem to make more sense. I'll look over these and make an order.

Thanks!

Paul

[innercityman]

Hi guys,

I'm about to start this build and I'd like to install the AL 20 vintage mirrored meters like the ones that Chandler use on their TG version. What resistor values need to be changed ?

Thanks for your help.

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