Hello everybody,
I'm relativly new to this stuff and also on the forum, so any help will be greatly appreciated.
I'm building VariMu compressor prototype, heavily based on Fairchild 660/670. I'm using a pair of 6BQ5's instead of 6V6's and (i know it's wierd :shock: ) Hammond mains power transformer (two 120V primaries in series to the plates, 50V bias tap on the secondary for the output, and 5V secondary for negative feedback (10%) :wink: , so it's basically around 5:1 ) for the control amp output and it works (gives me up to -80V DC CV). Although I don't know how it will perform compared to proper transformer.
For signal amp I'm using Edcor 600/10KCT on the input and Edcor 10KCT/600 on the output. I tried 4 pairs of 6BA6's in parallel, sounds fine, 4 6BC8's in parallel, that's OK too, but then I decided to try something different, something which is still in production. I remembered reading PRR's post about possibility of using 6L6's as VariMu tube and tried that. In triode connection, regulated +250V plate voltage, and control grids biased at -16V, and 500 Ohm balancing pot between cathodes with its brush going to the ground (cathodes are at around 6.6V), a matched pair of 6L6GC's gives 14dB of gain and 25+ dB of GR! Signal amp only draws around 50mA, signal and control amps together about 115mA. I'm using external Lamda regulated PSU.
The only problem is the amount of harmonic distortion (I know 6L6 was not designed for line level aplications). I'm running sine wave from signal generator in protools through digi001's crappy A/D/D/As to the input of the compressor and the output back to protools , and monitoring it in Waves PAZ Frequency Analyzer (not the best way to test things, but that's all I have).
With 1KHz sine wave at -20dB entering the compressor (with no GR) , output peak is at -6dB (in PAZ analyzer) , second harmonic is at -70dB.
If I change the frequency to 100Hz second and third harmonics are almost equal and at around -50dB. Lowering frequency to 20Hz increases distortion a lot (hard to see). Besides that the frequency response is strange. Transformers alone are flat from 20Hz to 20 KHz. But amp's response ref. to 1KHz is +4dB at 20Hz , +3dB at 30Hz , +1.2dB at 50Hz ,
+0.2dB at 100Hz and +0.3dB at 15KHz.
Finally my questions (tired of typing):
1.Is there any way to improve the distortion performance and frequency response ? (maybe different biasing :?: )
2.Does the VU meter in VariMu limiters show the actual amount of GR in dBs or just relative representation ?
I hooked up my GR meter like Fairchild's remote meter ( across 33 Ohm resistor and 10K pot for zeroing) but it doesn't show the amount of GR I can see in PAZ analyzer, it shows a lot more. For example, VU meter shows 5dB of GR , but change in level I can see in PAZ analizer is 0.6dB :shock: .Am I doing something wrong ?
Thank you very much.
I'm relativly new to this stuff and also on the forum, so any help will be greatly appreciated.
I'm building VariMu compressor prototype, heavily based on Fairchild 660/670. I'm using a pair of 6BQ5's instead of 6V6's and (i know it's wierd :shock: ) Hammond mains power transformer (two 120V primaries in series to the plates, 50V bias tap on the secondary for the output, and 5V secondary for negative feedback (10%) :wink: , so it's basically around 5:1 ) for the control amp output and it works (gives me up to -80V DC CV). Although I don't know how it will perform compared to proper transformer.
For signal amp I'm using Edcor 600/10KCT on the input and Edcor 10KCT/600 on the output. I tried 4 pairs of 6BA6's in parallel, sounds fine, 4 6BC8's in parallel, that's OK too, but then I decided to try something different, something which is still in production. I remembered reading PRR's post about possibility of using 6L6's as VariMu tube and tried that. In triode connection, regulated +250V plate voltage, and control grids biased at -16V, and 500 Ohm balancing pot between cathodes with its brush going to the ground (cathodes are at around 6.6V), a matched pair of 6L6GC's gives 14dB of gain and 25+ dB of GR! Signal amp only draws around 50mA, signal and control amps together about 115mA. I'm using external Lamda regulated PSU.
The only problem is the amount of harmonic distortion (I know 6L6 was not designed for line level aplications). I'm running sine wave from signal generator in protools through digi001's crappy A/D/D/As to the input of the compressor and the output back to protools , and monitoring it in Waves PAZ Frequency Analyzer (not the best way to test things, but that's all I have).
With 1KHz sine wave at -20dB entering the compressor (with no GR) , output peak is at -6dB (in PAZ analyzer) , second harmonic is at -70dB.
If I change the frequency to 100Hz second and third harmonics are almost equal and at around -50dB. Lowering frequency to 20Hz increases distortion a lot (hard to see). Besides that the frequency response is strange. Transformers alone are flat from 20Hz to 20 KHz. But amp's response ref. to 1KHz is +4dB at 20Hz , +3dB at 30Hz , +1.2dB at 50Hz ,
+0.2dB at 100Hz and +0.3dB at 15KHz.
Finally my questions (tired of typing):
1.Is there any way to improve the distortion performance and frequency response ? (maybe different biasing :?: )
2.Does the VU meter in VariMu limiters show the actual amount of GR in dBs or just relative representation ?
I hooked up my GR meter like Fairchild's remote meter ( across 33 Ohm resistor and 10K pot for zeroing) but it doesn't show the amount of GR I can see in PAZ analyzer, it shows a lot more. For example, VU meter shows 5dB of GR , but change in level I can see in PAZ analizer is 0.6dB :shock: .Am I doing something wrong ?
Thank you very much.