CBS AudiMAX II in the sick bay

[Boswell]

Was this with continuous sinewave input, and what input signal level and R1 setting were you using?

What were the corresponding generated control voltages (top of the 1uF cap)?

 

[audiokid]

I agree, I wish I could just add PDF's directly... is that possible @audiokid?

yes

 

[audiokid]

Testing the ability to add a PDF file in a post

 

[Link555]

Ok trying it out... Thanks!

 

[Link555]

Was this with continuous sinewave input, and what input signal level and R1 setting were you using?

What were the corresponding generated control voltages (top of the 1uF cap)?

I did not get as much time as I could have used last night but I did manage to run the sine wave through the unit and observe the voltage on the 1uf cap with respect to common.

Without any signal in the meter does this:

With a 1 Vrms (100Hz) differential sine wave going in and the threshold and release and attack fully counter clockwise the meter does this:

Yellow is the input sine and pink is the 1uf cap voltage:

 

[Link555]

With the threshold pot fully clockwise I get this (the 1uf Cap rms voltage was -12V):

With the threshold fully clockwise and the release also fully clockwise I get this (The voltage on the 1uF cap was -20Vrms):

 

[Link555]

Looking the photos just now I think I have wiring error on the attack pot...I think its wired after the 1uf not before it? I will confirm tonight.

 

[Boswell]

Looking the photos just now I think I have wiring error on the attack pot...I think its wired after the 1uf not before it? I will confirm tonight.

Does that come down to the same as the 1uF being on the wrong side of the attack pot (with pad resistor)? If so, then the 6AL5 anodes would be connected directly to the 1uF, which could produce very wrong results.

You show the input signal (100Hz, 1V rms), but do you know what waveform you have at the 6AL5 side of the 100nF feedback caps (both shown as C8)? It should be the full output transformer primary amplitude (one side of it) offset by the threshold pot voltage.

A control voltage of -12V d.c. (or rms!) could well be about right for that input, as the 6386 cathode voltage (and hence current through the valve) is about 20% of the quiescent. The curves for the 6386 show something of the order of a 10-fold decrease in transconductance going from a grid-cathode volts of -2 to -12.

 

[Link555]

Does that come down to the same as the 1uF being on the wrong side of the attack pot (with pad resistor)? If so, then the 6AL5 anodes would be connected directly to the 1uF, which could produce very wrong results.

From the picture I believe the 1uF is connected directly to pins 2and 7....which I will confirm tonight and fix. Not sure what I was thinking when that happened?

You show the input signal (100Hz, 1V rms), but do you know what waveform you have at the 6AL5 side of the 100nF feedback caps (both shown as C8)? It should be the full output transformer primary amplitude (one side of it) offset by the threshold pot voltage.

Thanks should be C8 and C9...

I did take a peak there with the meter before I inject the input signal, and saw the +265Vdc on the output transformer side, and on 63VDC on the other side. THe 63VDC dropped as the threshold knob was turned clockwise.

A control voltage of -12V d.c. (or rms!) could well be about right for that input, as the 6386 cathode voltage (and hence current through the valve) is about 20% of the quiescent. The curves for the 6386 show something of the order of a 10-fold decrease in transconductance going from a grid-cathode volts of -2 to -12.

Yes I think it's getting closer, but if I have the cap wired the on wrong side of the attack pot, that could explain some of my grief.

Thanks again Boswell your input is very helpful as always!

 

[Link555]

I found my wiring error and corrected it. Now the threshold, attack and release seem to be behaving better. The traces in the picture below are: Yellow- Input, one side WRT common, 500mV per division
Green- Output, one side WRT common, 10V per Division
Pink- Pins 2 and 7 of the 6AL5 WRT common, 20V per Division
Teal Blue- The top of the 1uF Cap WRT common, 10V per Division

With a 1 Vrms Sine Wave input

With a Square Wave input:

The meter is pretty much useless. I am debating reconfiguring the meter to be purely Db reduction meter.
Oh and the front panel arrived!

I also experimented with a variable output pad. When I cut off the output pot that was previously added, the noise floor dropped considerably. So that needed to go. I built a balanced T pad similar to what the Gates39B had on it. It worked, but at low levels the output transformer got loaded and I could see asymmetrical distortion happening. It would be funny to put an opamp buffer on the secondary and then add a dual gain pot through another opamp output buffer.... I am thinking about a Make before break switch with 12 T Pads to attenuate....

Also The dual gang input pot has worn out sections, I will replace the 30k pot with 100k.

 

[Boswell]

That's looking a whole lot better!

Your new front panel is very smart as well. You must have a some good metalwork and screen printing firms near you who can turn round this type of job in very quick time.

When I first saw the 100Hz scope picture, I was puzzled by the apparent phase advance shown in the output vs. the input trace. Then I saw the 5V offset between them, and the distortion in the output waveform. I'm assuming you are taking the output waveform from the secondary of the output transformer, so where's the d.c. offset coming from?

Some of the apparent output offset may be due to asymmetry in the waveform. This brought up something that I had jotted down in my circuit notes, and that was that the anode of the output triode is basically a current source driving the impedance of the output transformer, so the currents flowing through the diodes on peaks would subtract from and therefore distort the output waveform - you can just see that on the green trace. This may be what gives the unit its sonic character! Have you got any way of measuring distortion?

That apart, it would be good to see a long timebase scope picture of what the control volts do when the sinewave input amplitude is changed quickly from a low value to a high value, and a second picture doing the inverse. Medium settings on attack and release.

 

[Link555]

When I first saw the 100Hz scope picture, I was puzzled by the apparent phase advance shown in the output vs. the input trace. Then I saw the 5V offset between them, and the distortion in the output waveform. I'm assuming you are taking the output waveform from the secondary of the output transformer, so where's the d.c. offset coming from?

The output measurement is right on the the terminal strip, so on the secondary of the output transformer after H pad attenuator.. The input measurement was taken right on the output of the function generator.

Have you got any way of measuring distortion?

I can do a frequency sweep on my scope. Coupled with the function generator it has built in BODE function. That would highlight the harmonics, but I don't have proper audio analyzer like a Audio Precision.

I played with it a few times, still kind of a gimmick..... I do have a old HP spectrum analyser too. Its only good for audio frequencies.

That apart, it would be good to see a long timebase scope picture of what the control volts do when the sinewave input amplitude is changed quickly from a low value to a high value, and a second picture doing the inverse. Medium settings on attack and release.

I spent quite a bit of time playing with the controls with a pulse input, triggering on the positive and negative edge. I did not take any photos though. I will take some and post them.

Thanks Boswell!

 

[Boswell]

I don't think the swept audio function is of interest for the moment - that would be useful as a final test later. Your old HP audio spectrum analyser sounds like the right tool.

It would be a matter of doing separate spectrum pictures, firstly of the output of your generator, and then of the compressor output when fed with the same signal. Since the Audimax is a balanced circuit, you would expect only the level of the odd harmonics to show an increase in this sort of test. You may find that as you raise the input signal level to the point where one half of the 6386 vari-mu triode nears cut-off, some second harmonic comes in.

I'm impressed by what those Siglent scopes will do!

 

[Boswell]

I sat bolt upright in the middle of the night with this picture in my head. Why is it only half-wave rectifying?

 

[Link555]

Yes sir ... broken wire.. finally noticed that today too.
Yellow is input +
green is input -
Blue is the output of pins 2 and 7 on the 6AL5

In got the face plate installed and started to clean up the wiring.

Still not happy with the meter And need to make the mounting plate for the xlrs and TRS jacks...

 

[Link555]

Oh and for the output knob what do you think of this.... I don’t need to worry about the impedance issues of adding a pad this way...

 

[Boswell]

I don't think that will work - it's just lowering the loop gain inside the amplitude control loop.

 

[Link555]

I tried the output control out, it does reduce the overall output down to zero, But your correct it also affects the db reduction as well. Every control on this compressor interacts, so in using the ouput control like this it almost feels natural.

The meter is thing that bothers me, the ac signal is so small compared to the dc offset. The needle doesn‘t move with with audio. I am thinking about somehow amplifing the ratio of the ac signal to the dc signal on the meter ...

 

[Boswell]

It makes sense to have independent controls for input and output trims. The input trim is quite OK across the secondary of the input transformer, as that is outside the control loop. The output trim has to be on the secondary of the output transformer, as the feedback for the control voltage is taken from the ends of the primary windings.

From your scope shots, I was surprised by the amount of implied throughput gain. I have not seen an official specification for the Audimax II, but I have just found a BBC Research Department report on the Audimax II dated 1966, which is probably worth reading, that is if you can get past the BBC English and the liberal sprinkling of BBC prejudices.

Here's an extract from the report relating to the compression characteristics (R2 is the original threshold control on the input transformer secondary):

Here's a steady-state throughput graph:

The JLM mods will have changed all this, but you can get an idea of how the original design behaved, and, importantly, what the designed maximum output level was. You will have to add 6dB to this if you are running with it unloaded (dBu instead of dBm).

 

[Boswell]

The meter is thing that bothers me, the ac signal is so small compared to the dc offset. The needle doesn‘t move with with audio. I am thinking about somehow amplifing the ratio of the ac signal to the dc signal on the meter ...

I don't know what you mean by the "ac signal". The meter should show the response of the Audimax to whatever signal is being applied and not the actual amplitude of the input signal. The d.c. offset is there to set the quiescent position to the 0dB mark on the meter. The deviations about that should be the amount of applied expansion or compression around that quiescent point.

That said, I don't think the JLM mod will retain the original calibration of the meter away from the 0dB mark. It should go down below 0dB (and maybe up above 0dB) as the input signal level is changed, but it's unlikely that CBS would have originally calibrated the scale to the highly non-linear control characteristic of the 6386 vari-mu triodes.

You have the option of showing the 6386 cathode volts, as on the original, or the loop control volts as generated on the 1uF cap. These will give different non-linear scales in line with the wavy transconductance of the 6386. If you choose the cathode volts, it would be best to use the mean of the volts on the two halves of the 6386, as this will be a virtual signal ground, but still represent a version of the control volts.

Maybe with the compression circuit now behaving more-or-less as the JLM mods left it, you could do a couple of steady-state calibration runs, taking readings on your DMM while you step an input 1KHz sinewave from (say) -20dBu to +20dBu. One run would be looking at the average of the two cathodes (e.g. centre point of a temporarily-added pair of 4K7 resistors), and the other would be the (negative) control volts on the 1uF cap after the dual rectifier. I think that, once you have that evidence, you will be in a position to make an informed decision about how the meter should be connected in.