Let's try this again: converting SII to Power Amp for BeePre

[Deke609]

I started a thread yesterday about this, to which Tom-s and PB responded, but it disappeared. So here goes attempt #2. New or only partially answered questions in yellow.

But first:

IMPORTANT PUBLIC SERVICE ANNOUNCEMENT: The mods I have in mind are completely "unnecessary" and are just for fun and experimentation. If you turn the volume pot all the way up, or alternatively, if you have the Mourquiet upgrade, set the attenuators to 0dB attenuation, the SII will function as a power amp. And you can jumper the input selector switch to take it it out of the equation. Making the mods contemplated below will kill the resale value of your amp, and will likely limit the help that BH can provide if something goes wrong. (The foregoing is stated in the hopes that it obviates the need for PB, PJ or Doc. B to advise of same).

My plan:

(A) remove all three pairs of rca inputs, the input selector switch and the Mourquiet volume attenuators (and if I still had the vol. and balance pots instead of the MQ upgrade, I'd be removing those)

(B) install 1 pair of rca inputs in the mounting holes for the MQ attenuators towards the front of the amp (same holes used for vol. and bal. pots).

My Original Questions:

(1) Input resistors - what values and where?

          Tom-s responded (IIRC - please correct if wrong): 50K 1/4 or 1/2W tantalum resistors between each positive input and
          ground.

              My Follow-up Questions:

              (1-i) Does this result in a 50K input impedance or a 100K impedance (assuming that setting input impedance is one of their effects)?

              (1-ii) Is the 50K value optimal or are there other values that will work equally well? 50K tantalums are hard to find -- can I use anything
              between 50K and 100K or even 200K? (I anticipate that the answer depends in part on the answer to question (1-i) above)

(2) Optimization for BeePre: what if any mods would make the SII power amp optimal for use with the BeePre?  This may be covered by Question (1). Whereas the SII is stated to have an input impedance of 50K, I thought I read that the BeePre prefers to see 100K.

             PB's response: The BeePre is fine with any amp with an input impedance of 1.8K and up.

                    Follow-up question: Just to be clear -- while the BeePre may be happy with anything over 1.8K, is there an input impedance with which it is happi-est?

                Tom-s' unanswered question (IIRC): Can one put a 100K vol. pot in the SII?

MTIA,

Derek

[Tom-s]

1-i: 50K
1-ii:I'd search around 50K (Shinkoh 43K or 62K is also fine).

Follow-up question: I don't know. But can imagine, anything 1.8K and up.
My unanswered question: Yes, i think i can. Original stereomour had 100K.

Edit: In my SRII manual it shows 249K input impedance.

[Deke609]

Thanks Tom.

[Deke609]

I am going ahead with converting my SII-2A3 to a power amp for the Beepre, no longer just for kicks, but b/c I need the space at the front end of the amp to install the Audyn 3.3 uF True Copper Max caps - they are huge and heavy, much larger and heavier than the 4.7 uF ClarityCap CMRs I installed in the SII-45.

I will be removing the selector switch and the Mourquiet attenuators.

But I am still confused about what resistance values are needed/optimal on the input signal.  With the attenuators fully open, I measured 34.8 kOhms from the +ve rca inputs  to the output of the attenuators, and 134.8 kOhms from +ve rca inputs to ground.

This suggests 2 things to me:

(1) With the attenuators removed, the value of the resistor from +ve signal to ground should be 100K not 50K; and

(2) I may also need about 35 KOhms resistance on the +ve signal wiring itself (before the 100K reference to ground). Edit: But if this is "optional" or simply specific the implementation of the Mourquiet and otherwise unneeded, I will omit them.

@PB or @PJ: Are either or both of (1) and (2) correct?  If not, can you please tell me what resistances I need and where?

MTIA,

Derek

[Paul Birkeland]

You can put a 50K-470K resistor from each 12AT7 to ground.  Nothing else is necessary or helpful. 

[Deke609]

Many thanks PB. That answers the question of what value of, and how many, resistors to order.

I may have a follow-up question about adding the resistors to the 12At7s once I have the resistors in hand and am ready to install them.

[Paul Birkeland]

Here is a drawing of where they go (to the terminal strips, not the C4S PC board).

I'll just throw out there that a well made and fully upgraded Stereomour II should sell used for about the price of a new kit.  If you turn it into a power amp and you have lots of empty holes in the chassis and reduced functionality, I would expect the used value to drop by about half. 

Another far less destructive option would be to drill a couple of holes into the front of the wood base, then install a pair of RCA jacks on fender washers glued onto the wood.  You can wire each jack right into the terminals I've indicated on this drawing, then just leave the coarse control all the way up.  This would give you a direct pair of inputs, and if you changed your mind or wanted to sell your Stereomour, you could just purchase a new wood base and remove the extra wires.

[Deke609]

Here is a drawing of where they go (to the terminal strips, not the C4S PC board).

Many thanks PB. That's what I had in mind.

I'll just throw out there that a well made and fully upgraded Stereomour II should sell used for about the price of a new kit.  If you turn it into a power amp and you have lots of empty holes in the chassis and reduced functionality, I would expect the used value to drop by about half.

Yeah, but I think that ship has sailed already! See attached - a ton of copper braid shielding beneath electrical tape/shrink-tube and a kajillion green ground wires from the shielding, plus the impedance switch boards. 

I think I can use the Mourquiet/pot holes for a single set of inputs - but I might decide on adding two new holes closer to the terminal strips to make more room for the parafeed and coupling caps.  I'm not fussed about open holes -- more ventilation!

cheers,

Derek

[clarkmobley]

Considering this amp. To clarify, if using an extra direct set of rca inputs, would the 100k reisistors be necessary?

[Paul Birkeland]

Considering this amp. To clarify, if using an extra direct set of rca inputs, would the 100k reisistors be necessary?

There isn't enough information in your question to answer it.  How are you planning to wire the direct inputs?  Are they switchable?  If so, how will you be implementing that?

[clarkmobley]

Here is a drawing of where they go (to the terminal strips, not the C4S PC board).

I'll just throw out there that a well made and fully upgraded Stereomour II should sell used for about the price of a new kit.  If you turn it into a power amp and you have lots of empty holes in the chassis and reduced functionality, I would expect the used value to drop by about half. 

Another far less destructive option would be to drill a couple of holes into the front of the wood base, then install a pair of RCA jacks on fender washers glued onto the wood.  You can wire each jack right into the terminals I've indicated on this drawing, then just leave the coarse control all the way up.  This would give you a direct pair of inputs, and if you changed your mind or wanted to sell your Stereomour, you could just purchase a new wood base and remove the extra wires.

as described in this post. I will be using my own preamp, Sonnet Digital Audio Morpheus. If it could be wired with balanced  xlr inputs, so much the better, but not necessary

[Paul Birkeland]

Yes, you can wire for balanced inputs, but you'll need a pair of input transformers.

Yes, the resistors need to be there if you are removing the volume/balance pots and using one input.