Volume control preamp-o.p.t.

This idea was uttered in the JoeNet a few months ago but I never saw any practical follow ups. The intent was to combine the volume pot and load in a single component and thus remove another source of distortion and coloration. Another point I find interesting with this idea is that the volume control is not done before amplification, but after. This means that normally you would reduce a signal whereby the signal/noise ratio is raised and then amplified. This means you amplify the noise as well... The volume-opt amplifies the signal, then reduces it. This means that the S/N ratio is higher at the input and stays the same at the output of the amp. A great advantage if you are working with noisy components or small signal levels (preamps).

I decided to have a crack at it and asked my tranny source if this idea could be made into the real thing. The discussion ebbed into the background and I never really thought about it later. Then one day I received a package with just what I had asked for and a request to put the OPT through some tests.

Wow... My heart was beating faster at the thought of hooking it up and hearing it. This is a first test example, not even a prototype. An Elma rotary switch was quickly sacrificed to fit the 24 windings onto and a clip lead hook-up was made to get the experiment rolling. Each winding was 3dB apart from the next, impedance is 30K:25K and designed for the 26 (what else!).

OPT to the left, switch and tube to the right.

I am still experimenting with different loading schemes to get an optimum. I think after the results are in I will give feedback about the findings and perhaps ask if they can make a second version. I have high hopes for this idea...

Just some hints: The mids I have heard so far, on a not played-in opt, was trully excellent. I put on some jazz and the trumpet was sweet and warm, not as bright and sharp as before. The highs were not as extended as with the RC or choke load, but I think this is because of the way I loaded the secondary. I will experiment more with different load values etc to improve the design. So far I am very pleased with the results, and I know it will get even better.

I have tried numerous things to improve the circuit using the volume opt, but to no avail. I've spoken to some people who know a bit more about transformers and they also concluded that the volume-opt idea is not feasible. The coupling between primary and secondary at the smallest tap is so small that the signal collapses. This is what is resulting in the high loss of hf response and lower settings. At full output the coupling is about 1:1 and frequency range is indeed into the 80kHz mark, but when the switch is turned down, the hf drops quickly no matter what kind of load I use.

In conclusion, the idea is very good, there may even be ways to get it running properly, but the question will be is the end still justifies the means. My conclusion right now is that it doesn't...


Jim de Kort