[vinylb]

Mike i got it fixed !!!!!! I had the gain pots connected to the amp board reversed. This fixed both issues. I get a little music from unbalanced when switched to balanced but its very faint. No buzzing now. I recall you mentioned checking the ohms, I will do that. So just to confirm with the toggle lugs facing me
and the lugs are 3 rows of 2, 1st row, lug 1 goes to pin 2 on jumper, lug 2 goes to pin 2 on other jumper. Middle row of lugs, left lug goes to pin 3 to same jumper as lug 1 and middle, right lug goes to pin 3 on other jumper.

[mboxler]

Mike i got it fixed !!!!!! I had the gain pots connected to the amp board reversed. This fixed both issues. I get a little music from unbalanced when switched to balanced but its very faint. No buzzing now. I recall you mentioned checking the ohms, I will do that. So just to confirm with the toggle lugs facing me
and the lugs are 3 rows of 2, 1st row, lug 1 goes to pin 2 on jumper, lug 2 goes to pin 2 on other jumper. Middle row of lugs, left lug goes to pin 3 to same jumper as lug 1 and middle, right lug goes to pin 3 on other jumper.

"Sounds" right to me (pardon the pun).  Those gain pots are tricky...you have a 50/50 chance to mess it up, even if you think you have them right. 

So you have two preamps active at the same time playing different tunes? 

With the switch up (BAL), you should show an open connection from pins 2 and 3 on both J1 and J2.   With the switch down, you should get 0 ohms.

Good Luck!

Mike

UPDATE!!!  I just removed my jumper from pins 2 and 3, and got a 50K Ohm reading between the two!  I assumed it would be open.  Perhaps that is why you can get a faint signal though the unbalanced inputs...

[vinylb]

"Sounds" right to me (pardon the pun).  Those gain pots are tricky...you have a 50/50 chance to mess it up, even if you think you have them right. 

So you have two preamps active at the same time playing different tunes? 

With the switch up (BAL), you should show an open connection from pins 2 and 3 on both J1 and J2.   With the switch down, you should get 0 ohms.

Good Luck!

Mike

UPDATE!!!  I just removed my jumper from pins 2 and 3, and got a 50K Ohm reading between the two!  I assumed it would be open.  Perhaps that is why you can get a faint signal though the unbalanced inputs...

I only have one input active and found it odd that i could hear music when I switched. I tested it open and closed and get 0Ohms on both. I must be doing it wrong or testing the wrong points ?  I changed multimeter to lowest Ohm in my case its 200, took multimeter red and black nodes and attached it to the 2 leads coming from toggle and flipped switch.

[mboxler]

I only have one input active and found it odd that i could hear music when I switched. I tested it open and closed and get 0Ohms on both. I must be doing it wrong or testing the wrong points ?  I changed multimeter to lowest Ohm in my case its 200, took multimeter red and black nodes and attached it to the 2 leads coming from toggle and flipped switch.

Sorry...I misunderstood (I think).  You have a preamp hooked up to the RCA inputs only, and when the toggle is down (RCA), all is good.  When the switch is up (BAL), you can still hear faint tunes through the RCA inputs(?).

If my statement is correct, then my theory is that, even with no "jumper" between pins 2 and 3 on my amp, I'm showing a 30k to 50k resistance between the pins (not open as I had originally thought).  That may allow just enough voltage to be amplified and heard.  Probably BS, but a theory none the less.

As far as your multi-meter...with the toggle up (BAL), and measuring the resistance between the top "left" lug and the middle "left" lug, you should get no reading.  With the toggle down (RCA), you should get 0 ohms.  Are you saying that you get 0 ohms both ways?  That would mean a bad switch!

Wife's calling me...chat tomorrow.

Mike

Sun is up, coffee is kicking in.  Can you easily remove the connectors to pins 2 and 3 of J1 and J2 and measure the resistance between these bare pins?  Is there a possiblility that the two connectors are touching each other (that would also give you 0 ohms).  I know...I'm really fishing here!

Mike

[.Marcus.]

Thanks Marcus.

Are these the caps you've changed?  Looks like quite a fiddly job



Getting some higher quality caps on the PSU rails close to the chips is probably a good idea. I think I'm going to have to open mine up again and have a play.  While doing the changes did you do any direct listening comparisons between modded and normal boards?

Checking for EMI is not trivial, it may have a direct influence on audible noise but is likely to be more subtle and manifest in other ways.  A very fast and sensitive scope or analyser is probably needed, which I don't have.

The caps I changed for BGs were indeed the DC blocking between 1200 and 2092. 

BTW the data sheets for the 2092 and iraudamp7d make for interesting reading:
http://www.irf.com/technical-info/refdesigns/iraudamp7d.pdf
http://www.irf.com/product-info/datasheets/data/irs2092.pdf

cheers

Hi Richard,
thanks for point to the useful datasheets.

Yes these are the caps and you are right it is a little bit fiddly  But good prepearing of the cap pins and SMD Pads are the key   

I recap the boards and listen to it, first impression was that it wors, because the sound was a little bit harsh but after some time the caps settled.

What are the values of the tantals you replaced ?

I'm to be tempted to bypass them 

But i guess this is a bad idea in face of possible DC offset.

I will give a Elna Silmic a try or addition small high grade bypass cap.

Thanks,
Regards
Marcus

[.Marcus.]

Btw.. the caps named CP4A and CP2A of Figure 33 in Reference Design Sheet are used as PS Caps for the input stage of the 2092 and could be also replaced as mod ?

[RichG]

Hi Richard,
thanks for point to the useful datasheets.

Yes these are the caps and you are right it is a little bit fiddly  But good prepearing of the cap pins and SMD Pads are the key   

I recap the boards and listen to it, first impression was that it wors, because the sound was a little bit harsh but after some time the caps settled.

What are the values of the tantals you replaced ?

I'm to be tempted to bypass them 

But i guess this is a bad idea in face of possible DC offset.

I will give a Elna Silmic a try or addition small high grade bypass cap.

Thanks,
Regards
Marcus

The tatals were 10uF which I've replaced with 4.7uF.  Given that the following stage has an input Z of about 3K this gives a low cutoff of around 11Hz which I feel is adequate.  I wouldn't bypass them completely as the 2092 might show some DC at it's input which could cause problems, especially as their are no blocking caps on the 1200 stage. 

[RichG]

Btw.. the caps named CP4A and CP2A of Figure 33 in Reference Design Sheet are used as PS Caps for the input stage of the 2092 and could be also replaced as mod ?

Definitely candidates - just need to find them 

[.Marcus.]

The tatals were 10uF which I've replaced with 4.7uF.  Given that the following stage has an input Z of about 3K this gives a low cutoff of around 11Hz which I feel is adequate.  I wouldn't bypass them completely as the 2092 might show some DC at it's input which could cause problems, especially as their are no blocking caps on the 1200 stage.

Thanks for input.
Did you know how that additional 100K / 47K as used in reference and datasheet circuits between input and VS could impact the input impedance if used in SDS Design ?

[.Marcus.]

Definitely candidates - just need to find them 

If i find some time and mod the couple C's i will try to find them also

[RichG]

Thanks for input.
Did you know how that additional 100K / 47K as used in reference and datasheet circuits between input and VS could impact the input impedance if used in SDS Design ?

The 100k just allows a DC route to earth to stop pops when starting up.  Will have little impact on SQ if at all.

The 47k is the feedback resistor.  This is critical!  A higher quality resistor here 'may' have an impact, but you need to be really careful here as the whole stability of the amp will be effected by any changes here.  Personally I'd leave this well alone.  It's popular to change feedback resistors on conventional DIY amps but being Class D there well might be some serious side effects to changing this which are unpredictable.  The datasheet might give some pointers....

[.Marcus.]

The 100k just allows a DC route to earth to stop pops when starting up.  Will have little impact on SQ if at all.

The 47k is the feedback resistor.  This is critical!  A higher quality resistor here 'may' have an impact, but you need to be really careful here as the whole stability of the amp will be effected by any changes here.  Personally I'd leave this well alone.  It's popular to change feedback resistors on conventional DIY amps but being Class D there well might be some serious side effects to changing this which are unpredictable.  The datasheet might give some pointers....

Many thanks for explanation.

You are right, it should be leave well alone.

[vinylb]

Sorry...I misunderstood (I think).  You have a preamp hooked up to the RCA inputs only, and when the toggle is down (RCA), all is good.  When the switch is up (BAL), you can still hear faint tunes through the RCA inputs(?).

If my statement is correct, then my theory is that, even with no "jumper" between pins 2 and 3 on my amp, I'm showing a 30k to 50k resistance between the pins (not open as I had originally thought).  That may allow just enough voltage to be amplified and heard.  Probably BS, but a theory none the less.

As far as your multi-meter...with the toggle up (BAL), and measuring the resistance between the top "left" lug and the middle "left" lug, you should get no reading.  With the toggle down (RCA), you should get 0 ohms.  Are you saying that you get 0 ohms both ways?  That would mean a bad switch!

Wife's calling me...chat tomorrow.

Mike

Sun is up, coffee is kicking in.  Can you easily remove the connectors to pins 2 and 3 of J1 and J2 and measure the resistance between these bare pins?  Is there a possiblility that the two connectors are touching each other (that would also give you 0 ohms).  I know...I'm really fishing here!

Mike

With toggle in rca mode I get 0.02 ohms (mmeter set to 200ohms) on left and right, If I switch to Balanced I get 0ohms.

I'm going to take the toggle out of chassis to do check if on my wiring. I suspect too something with it.

[pslate]

I know one member briefly mentioned adding a Yaqin tube buffer, but I wanted to see what the general consensus would be on adding this to an SDS-254. This is the one I bought here on AC with Dodd power http://www.audiocircle.com/index.php?topic=100532.0

Obviously I'd love to add a Dodd buffer, but it's not in the cards from a financial perspective, so I was thinking the Yaqin CD3 in the hopes of adding a little warmth to my system.

Here is what I am running, I would be grateful for any input. Also, my system has to do double duty as an ht setup.

Source: 2012 Mac Mini Server hdmi
Pre: Harman Kardon AVR-254
Amp: SDS-254 for fronts, Emotiva UPA-2 for rears, strange combo but I am not to critical of rear channels as serious listening is in stereo.
Interconnects: Standard RCA, and 12 ga Monoprice
Speakers: Front Selah Verita Ported - Rear Selah Aura Jr. - Sub Selah TC12
 
Last two nights I had speaker dreams, I am a sick man, and I appreciate any help/therapy for my affliction 
Paul

[matt_garman]

Does anyone have a supplier and part number for the three-pin receptacle that connects the gain pots to the PCB?

On the build I did myself, I just pulled them off the pots included with the kit; I read someone else earlier in this thread stole them off old computer fans.

I now have a complete amp from Tom; I'd like to replace the pots with fixed resistors, but I don't want to ruin these pots; I'd like any mod I do to the complete amp to be trivially-easy to reverse.

[mboxler]

Does anyone have a supplier and part number for the three-pin receptacle that connects the gain pots to the PCB?

On the build I did myself, I just pulled them off the pots included with the kit; I read someone else earlier in this thread stole them off old computer fans.

I now have a complete amp from Tom; I'd like to replace the pots with fixed resistors, but I don't want to ruin these pots; I'd like any mod I do to the complete amp to be trivially-easy to reverse.

Sorry if I asked you this before.  Do you have any unwanted two-pin receptacles used for ON/OFF, LED's?  If so, I filed off the "tabs" on the back, and they will slid onto the two pins needed on the PCB.  Attach the resistor the same way you would have done with the 3 pin receptacle.  I've tried to find the same thing you are looking for...no luck.

Mike

[vinylb]

Does anyone have a supplier and part number for the three-pin receptacle that connects the gain pots to the PCB?

On the build I did myself, I just pulled them off the pots included with the kit; I read someone else earlier in this thread stole them off old computer fans.

I now have a complete amp from Tom; I'd like to replace the pots with fixed resistors, but I don't want to ruin these pots; I'd like any mod I do to the complete amp to be trivially-easy to reverse.

Hi Matt, yes I used my old connectors from computer fans. I have several types of molex connectors from the actual molex headers found on pcb to the 2 & 3 pin molex header connectors. Frys or Microcenter should have these. Another place that stocks them online is newegg.

http://www.newegg.com/Product/Product.aspx?Item=N82E16812189119

[vinylb]

With toggle in rca mode I get 0.02 ohms (mmeter set to 200ohms) on left and right, If I switch to Balanced I get 0ohms.

I'm going to take the toggle out of chassis to do check if on my wiring. I suspect too something with it.

I re-soldered the wires and i do not have any music bleeding through now but when I connect to Balanced I hear a slight buzz. I am ok with that for now since I only use RCA. But any suggustions would be helpful too

[mboxler]

I re-soldered the wires and i do not have any music bleeding through now but when I connect to Balanced I hear a slight buzz. I am ok with that for now since I only use RCA. But any suggustions would be helpful too

Not that I will know the answer, but when you say "connect to balanced", do you mean you are connecting a balanced preamp to the amp, or do you mean you you have an unbalanced preamp hooked up, but the toggle is in the BAL position?  Sorry for all the questions about your questions.

[vinylb]

Not that I will know the answer, but when you say "connect to balanced", do you mean you are connecting a balanced preamp to the amp, or do you mean you you have an unbalanced preamp hooked up, but the toggle is in the BAL position?  Sorry for all the questions about your questions.

Sorry, my bad for not putting enough detail. I only have rca connections from my Bottlehead Quickie preamp connected to the rca connection/inputs. When in that position, it sounds good. When I move the toggle to Balanced I get a slight buzz.

I also ran some polarity tests and found that I had my left and right inputs switched. I had this same problem on my CDA-254 kit. I guess my left and right is different to way Tom looks at it. I noticed the SDS chassis left and right inputs do not align to the left and right inputs on the amp board. From what I can tel Tom designed as if you were looking at the chassis from above and not from the rear...maybe I am just old, dazed and confused

I looked at Tom's pictures and he had it wired straight from amp board left to rear input right. I thought it was odd at time i assembled. I also noticed in his pictures he has his transformer wired so that  blue goes to AC-L which should be going to AC-N and Green going to AC-n which should be going to AC-L. Also saw that he has the power switch on side of chassis wired but could not see any molex headers or wires connecting to the right of amp board where its designated ext. power switch. Anybody else encounter this or anything else?


See pic below