[CYoung234]

Oops - too late - the case is already drilled for the pots. I guess from reading the thread that my take on it was that they did not affect the sound. Tom states they are high quality pots, and they seem to turn smoothly, etc. So, we will see. Thanks for the reply, though.

[guest1632]

Oops - too late - the case is already drilled for the pots. I guess from reading the thread that my take on it was that they did not affect the sound. Tom states they are high quality pots, and they seem to turn smoothly, etc. So, we will see. Thanks for the reply, though.

Hi, Well, hopefully the pot holes are in the back of the case. In that case, you can go online and search out for caps that will cover those holes. If you go back several pages you'll see what people have had to say about the pots and resistors.

Ray

[CYoung234]

Thanks for the input. I went back and looked through the thread, but could not find much suggesting that the pots were a problem. I will stick with them, and I will report back on my experience with them.

[walkern]

If you go back to the posts beginning on page 104 and read through you will find any number of posts suggesting that replacing the pots with fixed high quality resistors can offer a rather dramatic improvement in the sound of the SDS amps.  In my experience the naked Vishays worked some real magic.  It IS useful to use the pots initially to ascertain the 'ideal' amount of gain you want (depends on the type of preamp and speakers you will be using).  You can always leave the pots mounted in the chassis to fill the holes, and just leave them disconnected.

[CYoung234]

Thanks. I have read the entire thread through, before buying the amp, but I did not go back that far looking for info on the pots. I will take a look at it. Thanks for the advice. You are correct, once I get things dialed in, I can always put a fixed resistor in and leave the pots mounted.

[Barry_NJ]

Since there is no "Thread Search" function in this forum, here's a tip for anyone trying to find a specific bit of information, in a specific thread, especially one as large as this...

If you use the "PRINT" function in the menu at the top-right or bottom-right of the thread posts, it will open the full thread in one page as a text document. You can then use your browsers search function to find the key word you're looking for ie. "vishay".

Hope this helps.

using the method outlined above I quickly found this post, and many others following it...

--------------------------------------------------------------------------------
Title: Re: $175 Class D amp--120 wpc
Post by: walkern on 22 Oct 2010, 03:09 PM
--------------------------------------------------------------------------------

I had some left over 3 pin Molex connectors and decided to try substituting a single resistor for each input level control on my SDS 254 amp.  I tried 10 ohm Vishay metal film resistors first, and then switched to 4.7K ohm carbon film resistors  so I would better match the volume control output characteristics of my Pass DCB1.  The resistors both offered (a) a more 3D or holographic sound stage (2) more layering and extension both in front and to the rear of my speakers, and (c) a quieter background from which sound emerged (sounded much like adding good AC power conditioning).  The Vishay metal film resistors clearly tipped up the treble balance, and initially sounded a bit gritty or spitty or splashy, and they also tightened up the bottom end a bit.  The carbon film (generic... had 'em laying around from Radio Shack) resistors were smoother sounding with no emphasis in any particular region, but they did warm up the midbass a bit (fuller, but not bloated or tubby).  All in all I would say the improvement is as great as the difference between the CDA amps and the SDS amps, and all for less than $10.  The Molex connectors I used were not a perfect fit and had to be filed a bit (the locking system on them was different than Tom uses), but are snug and won't fall out.  Mouser part number for them is: 538-10-11-2033, and you'd also need crimp terminals for them that will attach to the legs of the resistors (that must be soldered IMHO) - Mouser part number 538-08-50-0114.

Highly recommended cheap and cheerful tweak that won't void your warranty.  I'm sure higher quality resistors would be worth trying.  The 10 ohm value leaves the input impedance similar to having the control pots fully turned up.  The 4.7K ohm value was equivalent to turning the control pots down by about 6 db.

Neil

--------------------------------------------------------------------------------

[CYoung234]

Barry_NJ, thanks very much! The lack of a thread search function was making for a lot of extra reading.

On another similar note, did people ever arrive at a consensus as to what resistors work the best? From what I have read, the Vishay "naked" TX2375 or TX2575 series seemed to be the one many were getting, but Walkern talked about Riken and Holco as well. Oh, and finally, where is everyone sourcing these from? Partsconexxion? Elsewhere?

[krikor]

Ditto thanks to Barry_NJ.  Great work around for searching. 

Regarding the pots. Tom does believe that contrary to their appearance, the Honeywell pots are very good quality and sound great (without directly negating the use of resistors, I get the impression he doesn't think they provide a significant improvement).  That said, I think its an easy enough tweak that it makes sense to implement once you've settled on a specific gain level and don't need to adjust it frequently.

I also wonder if the benefits derived perhaps come more from the elimination of those long leads carrying the signal to the pot and back again, going through all that "noise" inside the chassis?

I have not tried fixed resistors just yet, but plan to eventually.

[guest1632]

Ditto thanks to Barry_NJ.  Great work around for searching. 

Regarding the pots. Tom does believe that contrary to their appearance, the Honeywell pots are very good quality and sound great (without directly negating the use of resistors, I get the impression he doesn't think they provide a significant improvement).  That said, I think its an easy enough tweak that it makes sense to implement once you've settled on a specific gain level and don't need to adjust it frequently.

I also wonder if the benefits derived perhaps come more from the elimination of those long leads carrying the signal to the pot and back again, going through all that "noise" inside the chassis?

I have not tried fixed resistors just yet, but plan to eventually.

Hi, Well, don't know since I don't have one of these amps. Somewheres in this thread is a chart that has been repeated several times here. The 1.2 KOhm resistor will set the level to about the 27DB level which is supposedly standard for amps. I'd suggest you look for 1% resistors. Quarter to half watt should be ok. Yeah, maybe all that wiring coming from the pots to the connector may play a role here. When I someday order my amps, I will order them without the connectors, with the fixed resistors in place. one less mechanical thing that can possibly take away from the sound of the amp.

if the holes are already in the case, I'll look around for caps, or if it's the faceplate, i'll order one without the holes drilled.

Ray Bronk

[walkern]

Barry_NJ, thanks very much! The lack of a thread search function was making for a lot of extra reading.

On another similar note, did people ever arrive at a consensus as to what resistors work the best? From what I have read, the Vishay "naked" TX2375 or TX2575 series seemed to be the one many were getting, but Walkern talked about Riken and Holco as well. Oh, and finally, where is everyone sourcing these from? Partsconexxion? Elsewhere?

The Riken and Holco resisters were both nice, but once I got the naked Vishays I never looked back.  If memory serves I got them from Michael Percy... www.percyaudio.com.

Neil

[guest1632]

The Riken and Holco resisters were both nice, but once I got the naked Vishays I never looked back.  If memory serves I got them from Michael Percy... www.percyaudio.com.

Neil

Hi Neil,

What size of resistors and what values and how many did you get? What was the cost and shipping ETC.

Ray

[walkern]

The naked Vishays I got (TX 2352) were .6 watt rated... cost $12 each, and you need 2 for each stereo amp.  I don't remember what shipping was?  I chose a 4.75K value as that dropped the gain of the amp to where it worked best for my system (I have high sensitivity speakers, and don't listen LOUD anymore).  A 10 ohm resistor gives you max gain according to Tom.  Here is a chart with some other options for values to adjust the gain to your liking and some general instructions:

"For those who wish to change the pots for fixed resistors, here are resistor values to produce required gain. Gain is listed in dB: To change gain potentiometer to a fixed resistor, you would connect a fixed resistor between the yellow and black wires on the gain controls (of course, remove the gain pots). Leave the red wire disconnected.

Use high quality 1% resistors:

560 Ohm = 31 dB

1K = 30 dB

2.2K = 27 dB

5.2K = 23 dB

6.2K = 18 dB

7.5K = 16 dB"

[guest1632]

The naked Vishays I got (TX 2352) were .6 watt rated... cost $12 each, and you need 2 for each stereo amp.  I don't remember what shipping was?  I chose a 4.75K value as that dropped the gain of the amp to where it worked best for my system (I have high sensitivity speakers, and don't listen LOUD anymore).  A 10 ohm resistor gives you max gain according to Tom.  Here is a chart with some other options for values to adjust the gain to your liking and some general instructions:

"For those who wish to change the pots for fixed resistors, here are resistor values to produce required gain. Gain is listed in dB: To change gain potentiometer to a fixed resistor, you would connect a fixed resistor between the yellow and black wires on the gain controls (of course, remove the gain pots). Leave the red wire disconnected.

Use high quality 1% resistors:

560 Ohm = 31 dB

1K = 30 dB

2.2K = 27 dB

5.2K = 23 dB

6.2K = 18 dB

7.5K = 16 dB"

Hi, Ah yes, the chart. Thanks for the correction. Somehow, I thought it was a 1.2 Kohm resistor. Oh well, guess I'm getting old. lol.

Now since I haven't seen the board or the whole setup, is the classD three boards, one PS, and two separate board for each amp, or one board containing both channels. In order to operate this in bridged mode, you have to have two boards? is this correct? Sorry forgot how this all works.

Ray Bronk

[walkern]

I don't know which SDS amp kit you are putting together.  The 224, 254 and 258 are all single amp boards, (with one power supply board and 1 transformer).  The SDS 1000 is two amp boards (2 X 254), with one power supply board and 1 transformer.  You can check out photos of the single amp board models at Tom's website www.classdaudio.com

Neil

[srb]

I don't know which SDS amp kit you [Ray Bronk] are putting together.  .....  You can check out photos of the single amp board models at Tom's website www.classdaudio.com

No, he can't which is the reason he's asking.

Hi, Sorry because I can't see your [CYoung234] pics because of being blind .....

Steve

[guest1632]

I don't know which SDS amp kit you are putting together.  The 224, 254 and 258 are all single amp boards, (with one power supply board and 1 transformer).  The SDS 1000 is two amp boards (2 X 254), with one power supply board and 1 transformer.  You can check out photos of the single amp board models at Tom's website www.classdaudio.com

Neil

Hi Neil,

Thanks for the info. So $24 for two resistors? hmmm, gosh, just stick 'em in an envelope (softpack) and send them for almost nothing, well a few bucks anyway.

Ray

[maxitonus]

Hello gentlemen,
sorry fr my english but i'm a frenchie, very satisfied with my SDS470 feeding Duntech Prince speakers.

Now i want to try passive bi-amping, in order to avoid that the distorsion of the low frequencies may reach the medium-treble drivers, and assess whether the improvement is relevant or not really.

Therefore i need to keep the pots in order to allow to adjust separately the level for -low and -high frequencies.

This is why i have some questions, destined to those having tested passive bi-amping with four channels of amplifying.

1 Has somebody acquired some return of experience with passive bi-amping?compared results vis a vis the single amp?
2 Bi-amping is imposing to keep the pots (to allow for adjusting separately the level for lows and highs),.. therefore connect he 2 XLR symetrical (balanced) inputs in parallel..do yu know whether the position of each pot is modifying the input impedance or not? did anybody measure exactly which is the value of one channel's input impedance in balanced mode?(which will be divided by 2 when paralleled)
3 Is somedody having some return of experience with replacing the original pots by very good ones (ALPS for example)while shortening at minimum the wires connecting such pots to the board??results?

[CYoung234]

Okay, I finished my amp last night and took some measurements of the DC and AC out on the binding posts - they were all very good. The amp fired up perfectly. However, when I put it into my system - no sound. The gain pots are turned up 1/2 way. I have it set up for balanced operation but I am converting from RCA at my preamp out using some homebuilt cables. I tested the cables on my Bel Cantos, and they would not work at first - I had the pinouts as follows:

XLR Pin 1 - Shield, Pin 2, HOT (Blue), Pin 3 COLD (White) 0 the cable is Mogami W2549
RCA - Blue to the center pin, White and Shield to the body.

To get this to work with the Bel Cantos, I had to disconnect the shield from the RCA end.

At any rate, this has to be simple, as the amp powers up correctly. Oh, I had the J1 and J2 jumpers removed (actually I just had them on pin 3 so I would not lose them. Here are a few pictures showing the wiring. The gain controls were wired Black, White, Red from left to right with the control facing you and the pins facing DOWN.




And, the gain control wiring:




Later today, I will set it up for unbalanced RCA operation and try that. I rterminated the RCAs but did not terminate them at the amp - they are just taped off right now. Let me know if you have any other ideas or see something mis-wired.

[Nick77]

I think you mis-wired the pots. Looking from the back as you have it photographed the red wire should be on the right.

[CYoung234]

I think you mis-wired the pots. Looking from the back as you have it photographed the red wire should be on the right.

Nick77, thanks. I wired them per an email I received from Tom yesterday, but he may have been in a hurry or something. Here is what he said:

==============================================================

No... black on left, white in the middle, and red on the right.

-----Original Message-----
From: Craig Young [mailto:craig_young234@sbcglobal.net]
Sent: Friday, May 11, 2012 5:11 AM
To: sales@classdaudio.com
Subject: RE: Contact Form

Tom, thanks. Hey, one more question. The gain control leads are now Red, White and Black. If I am holding the gain control facing me with the terminals pointing DOWN, are the colors RED, BLACK, WHITE from left to right? Thanks,

========================================================

I recall someone had a picture showing this with the RED, WHITE and BLACK cabling, so I will track it down and double check it. Okay, I checked it (Post #2509 on page 126), and my wiring matches that.