Well, obviously in the volume control stage. It is either an incorrect component value or (less likely) a duff pot.
No, it isn't; the track resistance is irrelevant so long as it is truly linear with rotation. The only thing that can cause imbalance is if the wipers on the two sections are not aligned- nothing electronic can compensate for that.
Any pot of reasonable quality should work fine.
I would, and I did in the design. It is a 5k linear pot. I do not recommend you use any other value.
What's 'off-gain'? What's good about it?
This preamp uses only linear pots.
If you think they are easy solutions then I suggest you have a go at them. I suggest the active volume control is the solution.
This statement at least is correct. The tolerances in LDRs are enormous.
There is also the slight problem that they contain cadmium and are in most cases illegal to sell.
Unfortunately, there are no linear quality pots with 5k. For example, Alps has only 10k, 50k, 100k ...
I used 4k7 omeg green eco pots, supposed to be good and used in one of Doug Self's preamps The Signal Transfer Company: Precision Preamplifier but obviously not good enough
Make sure your wiering is correct, never swap 123 pin wires.
2is output. 3 is input and 1 is grnd.
Theres a possiblity that you swapped 3 and 2 wires in one of the pots. This happned to me when wiering a bournous 91 pot and adjustment was bad.
This would be ture if your pot is logramitch, but seems like they are using linear pots
I did, not your version, but that from B.Putzeys.
It too uses a linear volpot in the NFB.
It suffers the usual imbalance due to poor matching of the track resistances.
I converted it to switched and completely cured any audible imbalance.
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On this note I built the active volume control with 10K pot and 17dB max gain as per Mr Self's Small Signal book (fig 13.23).
Used Omeg eco green pot.
Yes it shows an imbalance at very low setting. One channel becomes audible first then the other . Increasing volume results in the channels balancing.
This is at very low settings and it's clear that I don't need the 17dB gain on tap so can reduce that sensibly.
I believe Omeg is not known for quality potentiometers. You would have to test the whole with ALPS pots. 17dB gain is absolutely necessary, since you have to amplify from -12dBu to 4 dBu.
Well I don't really need to test it as it's clear there's simply a gain offset with the pot position very low - first few mm.
I you need 17dB gain then you need it. But I don't where I've used the volume control as I generally have higher level sources and also don't want to be putting +4dBu level into a power amp where I have used it for that.
So in my case to be operating at a sensible mechanical position and range on the pot I need to bring down the max gain (or make it switchable).
Ah, I call that 'offness'. I don't consider it critical in a volume control; -80 dB should render most signals silent.
A mixer fader is of course another matter, and -100 dB or better essential.
It seems to me that the balance troubles some of you are reporting occur only at the very bottom end of the control travel, and signals are only audible at all because an excessive amount of gain is being used after the volume stage.
I have looked at my original notes from 1983, when I started using this form of active volume control, and the balance measurements were done over the gain range -20 to +26 dB. Very ordinary pots were within +/-0.3 dB over this range. The balance at very volume low settings did not seem too important, and was not measured.
Since the gain law falls quite steeply in the bottom 5% of rotation, this might well amplify the inevitable mechanical tolerances between the two wiper positions.
I suggest it makes sense to configure your system gain so that the volume control is well off the bottom in normal use.
Hi Carl, just reading about this great project, are there still any kits left?
Kind regards,
Hans
Kind regards,
Hans
I see that I am very late to the party here. I am interested in this project. You also mention that you were considering selling an assembled and tested version of your PC boards. I would really like to know if we can get boards or anything else on this project at this late date.
Doug Self Preamp PCB Kit
Of late I have experienced a flurry of people looking to purchase a Doug Self Preamp PCB kit. As result I have decided to order in the parts for yet one more limited round of purchases. Builders have all agreed that this is an outstanding project of great value.
A single PCB kit is $50 US plus the cost to ship. If you buy a 2nd kit it will ship at no additional expense.
The kit amounts to ...
#1) the main PCB
#2) the 5 x 1 IO and front panel PCBs
#3) The kit includes 2 (hard to find) OEM quad RCA phono connectors
#4) A Power supply PCB (120v but can be modded to 230v)
Once I receive your order I will send you a collection of documents that are a pdf the original Linear Audio article, project schematics and BOMs. All of the required parts to build are in stock at Mouser and Digikey. There are no exotic parts required to build.
If you are interested in buying one or more kits send me a private message and I will respond with more details. Kits are now in stock and ready to ship.
Of late I have experienced a flurry of people looking to purchase a Doug Self Preamp PCB kit. As result I have decided to order in the parts for yet one more limited round of purchases. Builders have all agreed that this is an outstanding project of great value.
A single PCB kit is $50 US plus the cost to ship. If you buy a 2nd kit it will ship at no additional expense.
The kit amounts to ...
#1) the main PCB
#2) the 5 x 1 IO and front panel PCBs
#3) The kit includes 2 (hard to find) OEM quad RCA phono connectors
#4) A Power supply PCB (120v but can be modded to 230v)
Once I receive your order I will send you a collection of documents that are a pdf the original Linear Audio article, project schematics and BOMs. All of the required parts to build are in stock at Mouser and Digikey. There are no exotic parts required to build.
If you are interested in buying one or more kits send me a private message and I will respond with more details. Kits are now in stock and ready to ship.