During the design of the GroupDIY bridge compressor, I've found that feeding the variable resistance tube with a CV often results in oscillation at various frequencies. In fact I got it stable at 1kHz only to find that it oscillated at 100Hz.
The oscillation is at about 4Hz which corresponds to the release time of the cap and res used: 0.25uF and 1M = 0.25s. I originally put this down to the fact that the amp I was using did not have a low enough source resistance to control it properly.
In fact I've just realised that the opposite is probably true! I have often wondered why compressor designers chose certain values for the timing cap, typically 0.1uF to 1uF. There is also the question of why it is not possible to reduce the attack resistor in some vari-mu designs.
Nearly all tube compressors operate on the feedback principle, that is to say, the output signal is rectified and returned to the vari-mu tube. This means that at a certain frequency, with fast attack times and a powerful amp, it is possible to charge the cap by individual cycles. This is obviously easier to achieve with low frequencies than high frequencies.
I have attached a chart showing the minimum source resistance required for various frequencies and typical cap values. This illustrates the problems in the bass with fast attack times. It may also explain why many compressors are designed for fixed cap/res values. It is also worth noting that more basic compressors, like the Altec 436 and Federal AM-864U, use a 1uF cap which makes less demands on the source resistance.
During the course of my experiments I found that a weak rectifier arrangement, like the Federal, was better at resisting bass oscillation than a pair of 6V6's driving diodes! I think I failed to understand these factors when I came up with ideal attack and release times. In fact it appears that it is the rectifier design that decides what is possible, not DaveP!
I have done this work using sine waves so a normal music programme may not exhibit the same effect.
I would like some confirmation/feedback on this please.
best
DaveP
The oscillation is at about 4Hz which corresponds to the release time of the cap and res used: 0.25uF and 1M = 0.25s. I originally put this down to the fact that the amp I was using did not have a low enough source resistance to control it properly.
In fact I've just realised that the opposite is probably true! I have often wondered why compressor designers chose certain values for the timing cap, typically 0.1uF to 1uF. There is also the question of why it is not possible to reduce the attack resistor in some vari-mu designs.
Nearly all tube compressors operate on the feedback principle, that is to say, the output signal is rectified and returned to the vari-mu tube. This means that at a certain frequency, with fast attack times and a powerful amp, it is possible to charge the cap by individual cycles. This is obviously easier to achieve with low frequencies than high frequencies.
I have attached a chart showing the minimum source resistance required for various frequencies and typical cap values. This illustrates the problems in the bass with fast attack times. It may also explain why many compressors are designed for fixed cap/res values. It is also worth noting that more basic compressors, like the Altec 436 and Federal AM-864U, use a 1uF cap which makes less demands on the source resistance.
During the course of my experiments I found that a weak rectifier arrangement, like the Federal, was better at resisting bass oscillation than a pair of 6V6's driving diodes! I think I failed to understand these factors when I came up with ideal attack and release times. In fact it appears that it is the rectifier design that decides what is possible, not DaveP!
I have done this work using sine waves so a normal music programme may not exhibit the same effect.
I would like some confirmation/feedback on this please.
best
DaveP
