Audio: Sunn Beta Preamp – Unclear Designs

[tav]

Hi all,

First time posting – I'm fresh into electronics, and I've hit some snags whilst copying one of the preamp channels from a Sunn Beta Lead schematic. (I've highlighted reference points for my questions.)

There's a number of questions I'm afraid, not so familiar with forum etiquette!

(Highlighted schematic attached is original one, and the other is my copy done in KiCad.)



Pink staggered line is one of the 2 channels for the preamp section, which is what I'm copying.

Question 01: There are capacitors that don't have uf/nf/pf multipliers, some are just ".0022" (e.g. C105 (in left green box)). Am I right in thinking this is ".0022F"?

--

The 4 caps in the green boxes have been a nightmare to find, so by recommendation from a friend I've swapped the following caps to (->) different values:

• C105 .0022 -> 2.2pf
• C106 .047pf -> 1pf
• C140 .005 -> 5nf
• C143 .027 -> 27nf

Question 02: How are these alternatives worked out? What super basic knowledge have I missed out on?

Question 03: There's an op amp (IC110) marked in red and blue that's shared between the 2 channels. For the one channel version that I'm making, should I still connect pin 8 to +15V and pin 3 to GND? If so, why? If not, why not?

Question 04: The op amp (IC109) before the previously mentioned one (IC110) doesn't have any reference to pins 4 & 8 being connected to the usual +/-15V, though the IC datasheet shows that those pins are reserved for +/-V. Should I connect these pins to the usual +/-15V or should I leave 'em disconnected (as the schematic seems to suggest )

See below for my copy of the schematic (omitting the FETs (e.g. Q103)).



Thank you all in advance!

[TimFox]

Question 1:  The old convention for capacitors in schematics:
A.  For non-polarized capacitors, all values > 1 are in pF (picofarads)
B.  For non-polarized capacitors, all values < 1 are in uF (microfarads)
C.  All polarized capacitors are in uF.
D.  All of the above:  unless otherwise specified

[srb1954]

C105 should be 2.2nF. A 2.2pF capacitor would pass a negligible amount of signal at audio frequencies.

The value of .047pF for C106 has to be a misprint on the schematic. A capacitance of this value is incredibly small - far less than what you would expect from normal stray capacitances between components.

It is hard to determine from the schematic what the value of C106 should be. If it was meant to be .047nF (47pF) then it also wouldn't have much effect at audio frequencies. If it was meant to be .047uF (47nF) then it would only have an effect at very low audio frequencies <50Hz. You may need to have a look at the circuit board to see what value is printed on the actual component.

When you have a capacitor in series with a resistor, like C105/R117, this acts as a high-pass filter, which passes signals above a certain cut-off frequency and progressively blocks signals below that cut-off frequency. The cut-off frequency can be calculated as
f = 1/(2*PI*R*C),   where the frequency is in Hz, resistance is in Ohms and capacitance is Farads.

For example, C105/R117 has a cut-off frequency of  f = 1/(2 * 3.14159 * 47*10^3 * 2.2*10^-9) = 1539Hz.

For the spare op amp connect the -ve input (pin 2) to the output (pin 1) like the section of IC109 connected to C140. The +ve input (pin 3) is then connected to ground and this places the op amp in a stable bias position so it doesn't draw excessive supply current or upset the operation of the other op amp in the package.

The schematic around IC109 is also incorrect - it definitely needs its power supply pins to be connected if it is to function.

All in all this seems a pretty dubious design. It looks as though it has been developed by trial and error design rather than a carefully engineered approach. In particular the circuit around IC104 and the tone controls around IC107 and IC109 seem to be a excessively complicated and potentially unreliable method of achieving simple functions.

[tav]

TimFox – That's a great bit of information, thank you!

srb – Thank you for such a detailed response. Your time is greatly appreciated! A friend of mine has the Beta Bass version of this amp; may take a look at its guts soon re C106.

I had no idea about the circuit of a high pass filter, as basic as it may be.

Any good pointers for books/self-contained resources that apply electronics knowledge to audio? It'd be great to be able to give back to the community at some point, rather than just leeching!

It doesn't surprise me that you say its dubious. I've been told these kind of amps birthed when solid state amplification was very new, so I'm sure designers would look back on these designs face-in-palm.

Thank you both again.

[srb1954]

Sorry can't suggest any reference texts for learning basic electronics. It has been more than 48 years since I started studying electronics so my information on the availability of basic text books is a little out of date.

I suggest you read through the posts on the beginners sections of the forum. There are probably plenty of useful suggestions there for learning resources.

[tav]

No problem, understood! Thanks!