GroupDIY Bridge compressor.
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ruairioflaherty
Well-known member
If this unit is still coming my way for a listen I might be able to get it measured on an Audio Precision if that's of interest?
ruairioflaherty said:
Hi Ruari,
That would be good, thanks.
I've been trying to contact Peter again with no luck. I think email must be so last century, its just not configured on todays devices. He originally agreed to test but now no response to arrange a date.
best
DaveP
I have attached the full set of schematics.
I know I'm tempting fate here, but I'll take a chance that it won't need anything radical done now!
This is not a project for a novice or a solder-by-numbers kit, it is something for guys who want a challenge and like p-p tube compressors.
You will notice that it does not have an input transformer like the original 26C, this means you will not need a pad as it does need quite a high drive level to achieve the maximum compression of over 17dB. Your pre-amp will need to have a maximum output of +20dB (7Vrms) or you will need to fit an input transformer instead.
On the positive side, this compressor cannot "thump" like a vari-mu as it is not push-pull and therefore never has a balance problem. The compressor uses a tube as a variable resistor to gradually reverse the phase of the transformer windings. The phases are clearly marked +&- and it will only work if these are strictly observed.
Parts list/Bom follows next.
good luck
DaveP
I know I'm tempting fate here, but I'll take a chance that it won't need anything radical done now!
This is not a project for a novice or a solder-by-numbers kit, it is something for guys who want a challenge and like p-p tube compressors.
You will notice that it does not have an input transformer like the original 26C, this means you will not need a pad as it does need quite a high drive level to achieve the maximum compression of over 17dB. Your pre-amp will need to have a maximum output of +20dB (7Vrms) or you will need to fit an input transformer instead.
On the positive side, this compressor cannot "thump" like a vari-mu as it is not push-pull and therefore never has a balance problem. The compressor uses a tube as a variable resistor to gradually reverse the phase of the transformer windings. The phases are clearly marked +&- and it will only work if these are strictly observed.
Parts list/Bom follows next.
good luck
DaveP
I've attached the parts list.
Most of these parts I've sourced in the UK, the heart of the amp is the two interstage transformers from "livinginthepast", they are £30 each. I think you have a far bigger choice of components in the US and equivalent parts will be fine. I've used 0.75W metal film because I can't get 1W metal film. I've used good quality polyester film caps throughout and Polyprop 630v caps on stage two where they are justified. There are a few silver mica caps where necessary. I would recommend using conductive plastic pots.
The tubes were obtained from Jim Cross at Vacuum tubes in Florida. I've used 6DT8's but you could use 12AT7's as long as you wire the centre tap properly, remember the heaters are elevated 43V!!! Do not put it down to ground!! You could also use 6V6's instead of the 6CM6's, but they will cost a lot more and sound the same.
No metalwork is included, you will probably have access to ready made boxes that I don't in the UK, I made mine myself because it was a prototype. The power transformer and power supply are completely optional. I used standard 55V types because they are a commodity, cheap and quiet; they are also very easy to mount when space is tight. You could use a tube rect. plus choke but it will cost more and take up more space. My Mosfet regulator is mainly to give a slow warm up time and it does that fine.
Any 1mA DC meter will do, you can re-scale it yourself as described earlier in the thread. If you want to change the attack and release timings and don't know how to do it, then you should not be making this project. I've used a busbar with the filter caps negatives returned to the relevent stage earths/grounds but a star ground layout will be just as good I'm sure. Good quality screened wire will be essential to isolate signals, the screens and the metal tube shells (Pin1) are the only things that should be grounded on the chassis, the busbar or star ground should only be attached to the chassis at one place near the power socket.
I've probably forgotten something but that will do for now.
Finally, don't forget, this project is for the forum funds, you are supposed to make a donation if you download these attachments and particularly if you make the compressor, how much you donate is up to you. I have given the rights to the design to the forum. I will post the specs probably next week. I will take sensible questions that can't be found elswhere but we would expect you to research exhaustively before asking basic stuff.
best
DaveP
EDIT: Added Inductor part # to BOM and price reduction on transformers.
Most of these parts I've sourced in the UK, the heart of the amp is the two interstage transformers from "livinginthepast", they are £30 each. I think you have a far bigger choice of components in the US and equivalent parts will be fine. I've used 0.75W metal film because I can't get 1W metal film. I've used good quality polyester film caps throughout and Polyprop 630v caps on stage two where they are justified. There are a few silver mica caps where necessary. I would recommend using conductive plastic pots.
The tubes were obtained from Jim Cross at Vacuum tubes in Florida. I've used 6DT8's but you could use 12AT7's as long as you wire the centre tap properly, remember the heaters are elevated 43V!!! Do not put it down to ground!! You could also use 6V6's instead of the 6CM6's, but they will cost a lot more and sound the same.
No metalwork is included, you will probably have access to ready made boxes that I don't in the UK, I made mine myself because it was a prototype. The power transformer and power supply are completely optional. I used standard 55V types because they are a commodity, cheap and quiet; they are also very easy to mount when space is tight. You could use a tube rect. plus choke but it will cost more and take up more space. My Mosfet regulator is mainly to give a slow warm up time and it does that fine.
Any 1mA DC meter will do, you can re-scale it yourself as described earlier in the thread. If you want to change the attack and release timings and don't know how to do it, then you should not be making this project. I've used a busbar with the filter caps negatives returned to the relevent stage earths/grounds but a star ground layout will be just as good I'm sure. Good quality screened wire will be essential to isolate signals, the screens and the metal tube shells (Pin1) are the only things that should be grounded on the chassis, the busbar or star ground should only be attached to the chassis at one place near the power socket.
I've probably forgotten something but that will do for now.
Finally, don't forget, this project is for the forum funds, you are supposed to make a donation if you download these attachments and particularly if you make the compressor, how much you donate is up to you. I have given the rights to the design to the forum. I will post the specs probably next week. I will take sensible questions that can't be found elswhere but we would expect you to research exhaustively before asking basic stuff.
best
DaveP
EDIT: Added Inductor part # to BOM and price reduction on transformers.
The Specification is attached.
Most of the specs are better than the original except the noise which is the same. It's not exactly the same because its more hum than noise, this is down to the design; having two interconnected transformers plus an inductor at the front end of your amp is never going to be quiet. Most of these pre-war compressors have poor noise specs, WE 1126, A322C & Presto 41A, all -45dB. Remember this is a non-feedback design.
It could be improved by ~6dB if the correction network after T2 was potted into a screened box, as this is where the 50Hz hum is mainly coming from. Farnell part# 118-1388.
My prototype does not have room for one now unfortunately, but I would definitely fit one if starting over.
This type of Compressor's controls operate a bit like an LA-2A, you can turn up the compression to suit, but you need the input control on full to get the full compression possible.
best
DaveP
Most of the specs are better than the original except the noise which is the same. It's not exactly the same because its more hum than noise, this is down to the design; having two interconnected transformers plus an inductor at the front end of your amp is never going to be quiet. Most of these pre-war compressors have poor noise specs, WE 1126, A322C & Presto 41A, all -45dB. Remember this is a non-feedback design.
It could be improved by ~6dB if the correction network after T2 was potted into a screened box, as this is where the 50Hz hum is mainly coming from. Farnell part# 118-1388.
My prototype does not have room for one now unfortunately, but I would definitely fit one if starting over.
This type of Compressor's controls operate a bit like an LA-2A, you can turn up the compression to suit, but you need the input control on full to get the full compression possible.
best
DaveP
While the rest of you were basking in the first day of a British summer, Jay and I were putting the Bridge Compressor through its paces at Dub Rek in Derby. He has some wonderful 15" concentric Tannoy monitors as big as fridges, you can hear every detail through them. [that would be British fridges not US ones!] ;D
Jay had some mixing to do for a local band on Pro-Tools. I was worried in case it did not come up to scratch because all I've ever put through it is sine and square waves. We tried it on drums first and it sounded good. Jay was able to compare the waveforms for different timings with Pro-Tools and seemed to think the top end was very well defined and smooth. There was no hint of thumping at all.
When he was happy with the result we moved on to bass and did the same, we ended up with 20ms Attack, 30mS Release, these settings seemed to bring it forward in the mix.
We then moved on to acoustic guitar and ended up with, I think 50mS/250mS, with the same result. At no stage was any noise detectable which was a great relief to me.
Finally we went to the fastest settings for lead vocal with 1mS/30mS all very smooth.
All this was a load off my mind after so much work, Jay will post his own report later. I asked Jay if he would like one and he said yes, a builder can't ask for more than that.
Thanks Jay for a great day and all your time and help, and your essential contribution to the Forum on this project.
best
DaveP
Jay had some mixing to do for a local band on Pro-Tools. I was worried in case it did not come up to scratch because all I've ever put through it is sine and square waves. We tried it on drums first and it sounded good. Jay was able to compare the waveforms for different timings with Pro-Tools and seemed to think the top end was very well defined and smooth. There was no hint of thumping at all.
When he was happy with the result we moved on to bass and did the same, we ended up with 20ms Attack, 30mS Release, these settings seemed to bring it forward in the mix.
We then moved on to acoustic guitar and ended up with, I think 50mS/250mS, with the same result. At no stage was any noise detectable which was a great relief to me.
Finally we went to the fastest settings for lead vocal with 1mS/30mS all very smooth.
All this was a load off my mind after so much work, Jay will post his own report later. I asked Jay if he would like one and he said yes, a builder can't ask for more than that.
Thanks Jay for a great day and all your time and help, and your essential contribution to the Forum on this project.
best
DaveP
letterbeacon
Well-known member
That's great news! Is there any chance we could hear some sound samples from the session at all? I'm intrigued!
rafafredd
Well-known member
WOW, eight tubes on this beast! Nice, but what are the two transformers T1 and T2 you have used in the bridge? Also, is the input really unbalanced? Might be good idea to add a bridging 10k:10k in the front, no? Just for studio standards... This have so many parts already that I don't think an extra trafo would break the bank. ;D
Rafa,
I've explained it elsewhere but if you look at the last schematic you will see that the interstages are connected in a certain way. When there is no CV the tube is a low impedance bridge between the windings, as the CV increases the tube is gradually switched off causing the windings to start going out of phase and causing cancellation. This process is all unbalanced but at a high voltage level ~10V.
The idea of this compressor is for guys to customise as they wish so an input TX is fine. I was confined a little for space because people wanted to keep it 3u.
The compressor is being converted to 115V and having labelling done this week and being shipped to Doug in the US next week.
Best
DaveP
I've explained it elsewhere but if you look at the last schematic you will see that the interstages are connected in a certain way. When there is no CV the tube is a low impedance bridge between the windings, as the CV increases the tube is gradually switched off causing the windings to start going out of phase and causing cancellation. This process is all unbalanced but at a high voltage level ~10V.
The idea of this compressor is for guys to customise as they wish so an input TX is fine. I was confined a little for space because people wanted to keep it 3u.
The compressor is being converted to 115V and having labelling done this week and being shipped to Doug in the US next week.
Best
DaveP
rafafredd
Well-known member
I see. I understand the bridge configuration with the two transformer, like the old Collins bridge compressor. I was just aksing what is the TYPE / BRAND / MODEL / SPECS of the transformers used in the bridge, so anyone could build one. I know you used EDCORs, but I can't see the part numbers in the pictures because they are inside the cans. In the schematics you labeled the output transformer, but not the bridge transformers, and it is also not in the parts list. I guess any pair of good quality 10k:10k would do, no?
Also, nice to see 6DT8 and 6BK7 used as these are not often used in modern audio boxes, so can be had for cheap!
Also, nice to see 6DT8 and 6BK7 used as these are not often used in modern audio boxes, so can be had for cheap!
rafafredd
Well-known member
No, it is not on the BOM or the schematics, but re-reading the thread, I have found out these are VT1399 interstage transformers from Valve Transformer in UK. I was under the impression that these were EDCORs, like the OPT, and that is why I couldn't find the info. Anyway, VT1399 for the masses. Never heard about those before, so one more for the list!
rafafredd
Well-known member
Thanks, and sorry for the confusion.
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