TwinLineAmp Opto Compressor TLA2A
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ruffrecords
Well-known member
OK, I think I have cracked it. Below is a table of output level versus gain reduction for the the VTL5C3 that I used in the RCA test bed using a 100K series resistor and negligible load:
It does not matter how much gain you use or whether you put it in the loop or before it, this table will always be true. So, you can use it backwards, with any value of gain to work out what the input level needs to be for any one of the values in the table. Then you can plot input level versus output level to see the shape of the compressor curve. As I suspected, the shape of the curve always remains the same. All that happens when you add gain is that the the input level corresponding to the threshold changes. As I mentioned before, the threshold occurs when the output reaches 1.6dBu so with 24dB of gain, for example, the input level at which compression begins is simply -22.4dBU. Reduce the gain by 10dB and the threshold rises by 10dB.
The bottom line is it does not matter if you put the pot before or after the Vactrol, all it does is alter the threshold. However, placing the pot before the Vactrol has a couple of advantages. First it means you can use a regular 10K pot as a load for the transformer. Secondly it means the voltage across the Vactrol resistive element is minimised which helps reduce distortion. So, I recommended using a 10K pot at the front and connecting the Vactrol direct to the TLA. I have updated the SchemeIt schematic to reflect this. I have also changed the series resistor value to 100K which is what I used in the RCA tests. I have also changed the LED drive from a single resistor driving both LEDs to one resistor for each LED. I am not certain this is necessary but I did it to ensure each LED is fed a defined current which should mitigate in the case where the LEDs have different turn on voltages.
Cheers
Ian
Code:
OP dBu GR dB
-8.25 1.25
+1.6 1.4
+5 8
+7 16
+9 24It does not matter how much gain you use or whether you put it in the loop or before it, this table will always be true. So, you can use it backwards, with any value of gain to work out what the input level needs to be for any one of the values in the table. Then you can plot input level versus output level to see the shape of the compressor curve. As I suspected, the shape of the curve always remains the same. All that happens when you add gain is that the the input level corresponding to the threshold changes. As I mentioned before, the threshold occurs when the output reaches 1.6dBu so with 24dB of gain, for example, the input level at which compression begins is simply -22.4dBU. Reduce the gain by 10dB and the threshold rises by 10dB.
The bottom line is it does not matter if you put the pot before or after the Vactrol, all it does is alter the threshold. However, placing the pot before the Vactrol has a couple of advantages. First it means you can use a regular 10K pot as a load for the transformer. Secondly it means the voltage across the Vactrol resistive element is minimised which helps reduce distortion. So, I recommended using a 10K pot at the front and connecting the Vactrol direct to the TLA. I have updated the SchemeIt schematic to reflect this. I have also changed the series resistor value to 100K which is what I used in the RCA tests. I have also changed the LED drive from a single resistor driving both LEDs to one resistor for each LED. I am not certain this is necessary but I did it to ensure each LED is fed a defined current which should mitigate in the case where the LEDs have different turn on voltages.
Cheers
Ian
ramshackles
Well-known member
It has been quite a while since I posted here, but I am still working on this...Although I'm starting to think I should've started with something 'easy' with the twin-line amp card like a preamp (I might just do that and put this on hold ).
I have built up the card, but not added the compressor parts yet. Before I do that I would like to test the card...there are a few things I need:
- Power supply...I have been holding out for the 'all-in-one' power supply you were working on Ian? Is that available yet?
- Which way round does the sowter line input transformer go? I might just be being daft, but it is not totally clear to me...
- How to test? The transformer is not connected to anything (yet) and neither are the two amp stages. Ive done all the 'passive' tests I can think of - checking power and grounds, resistances etc...anything else I can do?
I'd really like to make sure the amp stages are functioning correctly before adding the compressor parts..
I have built up the card, but not added the compressor parts yet. Before I do that I would like to test the card...there are a few things I need:
- Power supply...I have been holding out for the 'all-in-one' power supply you were working on Ian? Is that available yet?
- Which way round does the sowter line input transformer go? I might just be being daft, but it is not totally clear to me...
- How to test? The transformer is not connected to anything (yet) and neither are the two amp stages. Ive done all the 'passive' tests I can think of - checking power and grounds, resistances etc...anything else I can do?
I'd really like to make sure the amp stages are functioning correctly before adding the compressor parts..
Hi Ramshackles,
The info about the all in one power supply is inside the eztube lunchbox thread http://groupdiy.com/index.php?topic=53765.60
Ian sent me one to test but i haven't got the funds yet to buy all the components. I don't know how many of those pcb's Ian got for the test run and if he still has any, alternatively you could always get the HT350 board and get a seperate 12v regulated power supply for the heaters.
I can't remember the orientation of the transformer by heart but you could identifiy pin 1 of the sowter transformer and match the position using
the photo of the pcb to identify pin1 (S1) on the card.
Building the cards is pretty straight forward and i'm pretty sure they will work right away. The only thing i would make is sure is to double check the
twinlineamp config pdf about the different wire links and grid stopper resistors using the 6922 tube.
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/twinlineamp/
I'm looking forward to see your progress with the compressor. I'd like to try it myself as soon as i can get the optocells and the output transformers but that may take a while...
Regards!
Pierre
The info about the all in one power supply is inside the eztube lunchbox thread http://groupdiy.com/index.php?topic=53765.60
Ian sent me one to test but i haven't got the funds yet to buy all the components. I don't know how many of those pcb's Ian got for the test run and if he still has any, alternatively you could always get the HT350 board and get a seperate 12v regulated power supply for the heaters.
I can't remember the orientation of the transformer by heart but you could identifiy pin 1 of the sowter transformer and match the position using
the photo of the pcb to identify pin1 (S1) on the card.
Building the cards is pretty straight forward and i'm pretty sure they will work right away. The only thing i would make is sure is to double check the
twinlineamp config pdf about the different wire links and grid stopper resistors using the 6922 tube.
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/twinlineamp/
I'm looking forward to see your progress with the compressor. I'd like to try it myself as soon as i can get the optocells and the output transformers but that may take a while...
Regards!
Pierre
ruffrecords
Well-known member
@Ramshackles,
The all in one power supply PCB is available. PM me your address and I'll send you one FOC.
I don't know which way the Sowter 10K:10K transformer goes in. I got my first two in the post a couple of days ago. One side had was looks like a marker pen dot by the centre pin. I'll find out.
For testing I usually fit a 10K preset pot at gain1 and gain2. If you set them to about half way you should get around 20dB of gain. You need to run twin screened wired from L+,L-, LS and from M+, M- and MS to the pins L_, L- and M+, M-. Temporarily link TO1 to InN1 and link TO2 to IN2 to connect the transformer outputs to the amp inputs. Feed a balanced signal into the mating edge connector and you should be able to see the signal emerge at OUT1 and OUT2 on the edge connector.
Don't forget there are very high voltages present on this board. Take care! Do not leave out RHT. This is responsible for discharging the on board decoupling capacitor if you unplug the PCB.
Cheers
Ian
The all in one power supply PCB is available. PM me your address and I'll send you one FOC.
I don't know which way the Sowter 10K:10K transformer goes in. I got my first two in the post a couple of days ago. One side had was looks like a marker pen dot by the centre pin. I'll find out.
For testing I usually fit a 10K preset pot at gain1 and gain2. If you set them to about half way you should get around 20dB of gain. You need to run twin screened wired from L+,L-, LS and from M+, M- and MS to the pins L_, L- and M+, M-. Temporarily link TO1 to InN1 and link TO2 to IN2 to connect the transformer outputs to the amp inputs. Feed a balanced signal into the mating edge connector and you should be able to see the signal emerge at OUT1 and OUT2 on the edge connector.
Don't forget there are very high voltages present on this board. Take care! Do not leave out RHT. This is responsible for discharging the on board decoupling capacitor if you unplug the PCB.
Cheers
Ian
ramshackles
Well-known member
thanks. If I have time tonight I will try all the above.
Yes, IIRC, my 10K:10K transformer has a dot on one side, I'll check tonight.
Yes, IIRC, my 10K:10K transformer has a dot on one side, I'll check tonight.
ruffrecords
Well-known member
G-Sun said:
The EZ Tube Mixer and Lunchbox are 100% tube designs. I have no plans to design a DTOA.
Cheers
Ian
ramshackles
Well-known member
My sowter transformer has just 2 pins on one side, rather than 3. Judging by this diagram:
http://www.sowter.co.uk/specs/3293.htm
I'd say that they are pins 1 and 3 then...so thats my orientation.
PSU arrived; thanks! I will start looking at that.
http://www.sowter.co.uk/specs/3293.htm
I'd say that they are pins 1 and 3 then...so thats my orientation.
PSU arrived; thanks! I will start looking at that.
ruffrecords
Well-known member
G-Sun said:
You may be right. I did a Google search for it and only got hits for a discrete op amp. Do you have a link to the D-AOC?
Cheers
Ian
ruffrecords
Well-known member
G-Sun said:
Ah! Now I understand. TLA2A is very similar in concept to the D-AOC. It would use the same sort of opto device and would have amplifiers in more or lees the same places. I am a little concerned about the way the opto LED is driven in the D-AOC and also the way stereo linking is achieved but other than that they are much the same as any opto compressor.
I did do a paper design for a very simple stereo optical compressor based on a single TLA card but it has not been built of tested.
I have been asked many times if I am going to do a tube compressor and the answer is 'eventually' when I finish doing everything else!!!
Cheers
Ian
L´Andratté
Well-known member
I have no illusions about the extent of my knowledge, compared to basically everybody else on this forum, but as optical compression is one of the areas I dug somewhat deeper, let me make these remarks:
- I have tested all VTL5Cx as well as NSL32(-sr3). I can only agree to what a previous poster said about the usefulness of the often blurbed about inherent time constants of the optos. Not very useful for a wide range of audio sources. Bad distortion with low frequencies, borderline fuzz. Two resistors and a 470n cap could help. But you dismissed that with nonchalance, so maybe there´s a different approach? I´d like to know!
- the circuits I first built had the bad property to go from not compressing to a lot compression when crossing threshold. Very annoying. To get a soft transition I had to bias the LED *a little bit* ON. This is very helpful thing that I wouldn´t leave out anymore.
- I want to proclaim :
that a -3dB-slope hp-filter in the sidechain gives an even compression response to all freq., it´s really a different level of usefulness (to me) and so easy to do. The usual 1 pole hp is just an additional uneven reponse to the one of no filtering at all.
And (again) my thanks and best wishes to all of you out there!
- I have tested all VTL5Cx as well as NSL32(-sr3). I can only agree to what a previous poster said about the usefulness of the often blurbed about inherent time constants of the optos. Not very useful for a wide range of audio sources. Bad distortion with low frequencies, borderline fuzz. Two resistors and a 470n cap could help. But you dismissed that with nonchalance, so maybe there´s a different approach? I´d like to know!
- the circuits I first built had the bad property to go from not compressing to a lot compression when crossing threshold. Very annoying. To get a soft transition I had to bias the LED *a little bit* ON. This is very helpful thing that I wouldn´t leave out anymore.
- I want to proclaim :
that a -3dB-slope hp-filter in the sidechain gives an even compression response to all freq., it´s really a different level of usefulness (to me) and so easy to do. The usual 1 pole hp is just an additional uneven reponse to the one of no filtering at all.And (again) my thanks and best wishes to all of you out there!
L´Andratté
Well-known member
Hey, I hope I didn´t choke this thread to death with my smart ass comments :
Just in case, I´m bumping it now. I hope I didn´t come over as wanting to teach electronics lessons *lol*
My statements above were rather aimed at spawning discussion about these points as my experience is mainly based on ss optical compressor and noise gate-builds for bass, guitar, synth. My personal stereo(M/S) optocomp has not yet left the breadboard...
E.g. my favouring the -3dB Filter may be based on processing bass heavy stuff. There´s one thread on this forum, where noone else than Ted Fletcher dismisses it as overcomplication... ;D
Always interested in opinions...
Carpe diem everyone!
Just in case, I´m bumping it now. I hope I didn´t come over as wanting to teach electronics lessons *lol*
My statements above were rather aimed at spawning discussion about these points as my experience is mainly based on ss optical compressor and noise gate-builds for bass, guitar, synth. My personal stereo(M/S) optocomp has not yet left the breadboard...
E.g. my favouring the -3dB Filter may be based on processing bass heavy stuff. There´s one thread on this forum, where noone else than Ted Fletcher dismisses it as overcomplication... ;D
Always interested in opinions...
Carpe diem everyone!
ruffrecords
Well-known member
Sorry, I seem to have missed the last two posts.
The various optos do indeed have different attack and decay time constants. If you want to emulate an LA2A then it is probably best to use one with relatively long time constants. On the other hand, if you want to be able to set the attack and release time then you need to make sure the opto is fast enough that it is not the limiting factor.
If you get a lot of sudden compression the there are three possible reasons - either the LED driving circuit is poor or the top resistor of the pot divider formed by the input resistor and the opto is too small or you have no diode carrying the reverse phase signal (this caused the coupling cap to charge up like a charge pump). The optos have several 'regions' you can operate them in, each of which produces a different ratio.
HP filtering in a side chain is useful if you have relatively short attack and decay times as it prevents the compressor trying to 'follow' low frequency content.
Cheers
Ian
The various optos do indeed have different attack and decay time constants. If you want to emulate an LA2A then it is probably best to use one with relatively long time constants. On the other hand, if you want to be able to set the attack and release time then you need to make sure the opto is fast enough that it is not the limiting factor.
If you get a lot of sudden compression the there are three possible reasons - either the LED driving circuit is poor or the top resistor of the pot divider formed by the input resistor and the opto is too small or you have no diode carrying the reverse phase signal (this caused the coupling cap to charge up like a charge pump). The optos have several 'regions' you can operate them in, each of which produces a different ratio.
HP filtering in a side chain is useful if you have relatively short attack and decay times as it prevents the compressor trying to 'follow' low frequency content.
Cheers
Ian
rmaier
Well-known member
It's been a while since anyone has posted on this topic but I'd really like tor try building a pair of Twin Line Amp opto compressors. I've been through this thread a number of times now, as well as its counterpart "Tube Compressor" thread. I'm trying to get my head around the various schematics of potential build versions. Ideally, the build would be 2 channels, each built around its own TLA PCB and dedicated opto pairs, along with G.R. meters and the ability to stereo-link. I hope I'm not muddying the waters by including what appears to be a recent (? - 2016) version of the poorman schematic. Considering what I'm hoping to build, is this attached version the best one to start to base my build on? Does it represent current/improved thinking on the TLA in a compressor or do the versions on this thread better suit what I'm after?
Thanks!
Ralph
Thanks!
Ralph
