Hello
I'm building this preamp inside a roberts Akai unit.
I built using parts on hand that were close to the needed values and it worked very well, and sounds good, so I bought all the correct bits.
However I have a couple questions if someone doesn't mind indulging me.
Why is the output cap so large (3.3uf) when the cap between the stages is comparatively small (.02) ?
I have some .22 high voltage orange drops needinga home. Coul they be used in the output application?
And is there any advantage to using a larger cap (say .1) between the stages?
Thanks
I'm building this preamp inside a roberts Akai unit.
I built using parts on hand that were close to the needed values and it worked very well, and sounds good, so I bought all the correct bits.
However I have a couple questions if someone doesn't mind indulging me.
Why is the output cap so large (3.3uf) when the cap between the stages is comparatively small (.02) ?
I have some .22 high voltage orange drops needinga home. Coul they be used in the output application?
And is there any advantage to using a larger cap (say .1) between the stages?
Thanks
The output cap is sized according to the load, which often would be the volume control.
Tube circuits would have 100k to 250k controls, and so the source should have 1uF to 0.47uF output caps.
Solid state circuit volume controls can be as low as 10k, and so need around 10uF caps.
The interstage caps see typically 0.5M to 1M loads, and so can be much smaller in value.
Do not change the value of the interstage caps (or the one in the feedback loop) much, if at all,
or there could be low frequency instability, or subsonic peaking.
Tube circuits would have 100k to 250k controls, and so the source should have 1uF to 0.47uF output caps.
Solid state circuit volume controls can be as low as 10k, and so need around 10uF caps.
The interstage caps see typically 0.5M to 1M loads, and so can be much smaller in value.
Do not change the value of the interstage caps (or the one in the feedback loop) much, if at all,
or there could be low frequency instability, or subsonic peaking.
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There are some obvious weaknesses in the design that could be improved. (But then nearly all designs have a weakness or two.)
1. 0,022uf is used with a 1 meg grid resistor, which forms a high pass filter. The CR time constant is not nearly enough for
proper bass response. The fundamental circuit design is basically a low frequency oscillator with losses included, CR etc, to stop
the oscillation. Kind of an oxymoron. Positive feedback occurs at some very low frequency through the common
power supply for both stages. IF you are using subwoofers, they should help with the low frequency response (FR).
0,22ufd might lessen the losses enough so as to start the oscillation. I would be careful. Not a good idea imo.
2. The "gain"/negative feedback control is also a small "semi tone control', a little frequency selective but not horrible. Again,
subs would help. I would sub a high quality poly cap in place of the 22uf polarized cap in the feedback loop.
3. The 3.3uf is large enough to accommodate a moderate to high input impedance (Z) amplifier without much bass loss. I would not
go lower in ufd value if you wish good bass response, unless the electrolytic capacitors fattens the bass enough for your tastes.
(Hence, tubes known for fat bass with electrolytics.)
Probably best to leave the circuit alone, especially if you use a sub(s).
cheers and good fortune on your quest.
pos
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HI THanks for your response. In this instance this is being used as a microphone preamp for recording, rather than a hifi pre in front of a power amp.
I'm curious how you'd change your assessment, if at all, with that perspective.
I'm curious how you'd change your assessment, if at all, with that perspective.
Well, it depends upon
1. how flat of a frequency response you want though the entire audio range?
2. how good the musical quality you want?
Is this Dave a "recording engineer" or just hobbyist, not that it makes much difference?
cheers
pos
1. how flat of a frequency response you want though the entire audio range?
2. how good the musical quality you want?
Is this Dave a "recording engineer" or just hobbyist, not that it makes much difference?
cheers
pos
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One could argue that the interstage HP pole needs to be where it is to be dominant, with worst-case 10K/22uF pole in the feedback path, only a decade away. Also that feedback through common supply impedance over two stages is negative.
All good fortune,
Chris
It is not strictly 180 degrees, but varies widely. We don't see the rest of the PS design; how good or poor. Other networks
involved with the power supply.
Could be clean but I would not count on it without further exploration, especially with a larger coupling capacitor.
Using a high pole will negatively affect the musical quality through the highs. (Hopefully you are using a good mic)
Polarized caps deteriorate sonic quality and alter the bias of the input tube. Changing to better tubes would require a
whole re design.
OP, please never use a copper lead Orange Drop. Mundorf makes an accurate M EVO in oil aluminum capacitor
for B+ supply that is excellent. It is more expensive though, and physically large. Small caps might be thin though.
The question remains, how good do you want the preamp to be?
Cheers
pos