Reducing the output of the Stereomour

[ebag4]

Seems like an odd topic for a 3.5 watt amp, doesn't it?

I have determined that my Stereomour (built as a power amp) sounds the best being driven directly by my DAC.  This means that I am adjusting the volume digitally using MPD/ALSA on a dedicated Linux single board computer.  When you use the digital volume control with a 16 bit DAC you loose resolution, the more the volume is attenuated the more resolution you loose.  

What I would like to do is reduce the volume out of the Stereomour to something hovering around the maximum level I listen at when the MPD/ALSA volume control is at 100%.  this configuration would allow me to listen at my normal levels and still have the full resolution of the audio signal.

I currently have a pair of 100K Caddock resistors across the connections the volume control would normally connect to.  It seems to me that I should be able to replace these resistors with different values to achieve the reduction in volume.  However I am not certain that I am thinking correctly because I am already using 100K resistors, it seems that a greater resistance would be required to reduce the output but I believe the volume control has a maximum resistance of 100K so it seems a bit counter-intuitive.  Where am I going wrong?

My goal is to decrease the output about 15dB.  Any recommendations are appreciated.

Best,
Ed

Edit: Thinking this through a little further the 100K is the resistance to ground of the audio signal, so reducing this resistance will allow more of the signal to sink to ground thereby reducing the volume....I think... ;-)

Assuming that is correct, can anyone tell me what resistance I would need to use to drop the output by about 15dB?  I don't think this comes into play but my speakers are 97dB 1w/m.

[Grainger49]

Ok, I'm a fan of KISS.  How about installing the volume pot that came with your kit?  Alternatively you can make an S5 stepped attenuator that puts only 2 resistors in the signal path at a time unlike most stepped attenuators.

http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html

VoltSecond knows his stuff.

[kgoss]

I'm sure Paul Joppa can calculate the value you want mathematically, but he has also suggested people install the pot like Granger suggests above.  Then after you adjust the gain to whatever you prefer, just measure the resistance of the pot and replace it with the same value resistors.

[ebag4]

Thanks for the input guys, in the end installing the volume pot made the most sense.  I found the pot, installed it(temprorarily), and to my surprise I am not detecting any degradation of the signal!  I only have a few minutes on it but so far so good.  I may have to install it permanently but the way it is installed now it is tucked under the back of the amp, I adjusted it to the level I wanted with the MPD/ALSA volume control at 100% and VIOLA, I can now adjust the volume down a bit with the MPod volume control on the iPod Touch remote control as well as select music, build playlist, etc..

Best,
Ed

[ebag4]

Well, I pulled the volume control back out and put the 100K Caddocks back in, big difference!  I didn't notice the difference when I put the volume control in but when I removed it the difference was obvious, the bass is back in spades (sub amps fed high level from the Stereomour) and the music has returned with it's ability to draw you in, dynamics, etc and all of the audiophile jargon, basically it sounds a lot better.

Now if I can get rid of the slight low level non volume dependent hum/buzz from my Tranquility SE I will be set.  The iPod input I have set up for my son doesn't have this issue, dead quiet.  I have tried eliminating the AC ground from the Tranquility, that didn't have any impact.

I am also still trying to come up with a way to remove a bit of the gain so that I can run MPD at near 100% volume (direct connect from the Tranquility to the Stereomour).  I tried wiring the Stereomour for 45 use with the 2a3s, didn't work out too well, now it seemed to clip at low levels, my smallish (in value not size) parafeed caps could have been an issue.  So I have gone back to 2a3 wiring.

Best,
Ed

[Grainger49]

And now you are ready for an all Caddock S5 attenuator as linked above.

[Paul Joppa]

You could substitute a 12AU7 for the 12AT7 and get 11dB gain reduction. You'd have to change the shunt reg part of the board - use 2490 ohms for R3, and 6.49K for R4 (replacing the jumper).

[ebag4]

You could substitute a 12AU7 for the 12AT7 and get 11dB gain reduction. You'd have to change the shunt reg part of the board - use 2490 ohms for R3, and 6.49K for R4 (replacing the jumper).

Hi Paul, I meant to thank you for this recommendation and got sidetracked.  This would be a nice solution because I wouldn't have to add anything to the audio chain, my reservation is that I have a Pavane 12AT7 that I really like.  I think I may try a pair of those RCA format in-line attenuators, they are inexpensive and are available in a few different values.

Thanks again,
Ed

[ebag4]

I had some time today so I decided to start experimenting with various values of resistors in line with the signal.  I tried a number of values and ultimately landed on 100K.  This allows me to get in the 100dB range (song dependent of course) with the digital volume control of MPD at 100%.  I can make minor volume adjustments in the digital realm without throwing away too many bits.  I will order a pair of Caddocks to replace the ratshack resistors I have in there now.

My signal chain is now Linux single board PC> Tranquility SE DAC> Stereomour/Bass Amps>V1 Speakers

While I was at it I decided to highpass the Stereomour at about 150Hz using a high quality Sonicap Platinum cap.  To make this work I had to move from a high level feed to the bass amps to a low level feed.  I pulled this off at the switch I use to switch between my 2 inputs.  In the past when I have tried to use a "Y" connector to connect the bass amps and the Stereomour it resulted in a  aggravating hum.  To remedy this I decided to run the low level feeds to the bass amp through 3.3uF Clarity caps, I also ran a 100K resistor from ground to the signal feed, the result is clean bass without hum.  

Hooked it all up, everything sounds good and no hum, I love DIY.

Best,
Ed  
                                                                                    

[earwaxxer]

ebag - that sounds like tweaking with a capital T! That takes a healthy pair of cojones!

[ebag4]

ebag - that sounds like tweaking with a capital T! That takes a healthy pair of cojones!

Thanks earwaxxer, but these mods aren't really difficult or extreme but they do work out for my setup rather nicely.  That is the beauty of building your own amp, I would be far more hesitant to start modding a retail product of this value but since I am familiar with this build I am much more confident that I can fix anything I may break.

Best,
Ed