[ScubaSteve87]

FINALLY I just got the opa551's for the millet so it is up and running. I am using it through the onboard optical out on my comp and the Lite DAC-AH. My headphones are the AD2000. first of all the gain on the output of the dac-ah is SOOO high. I seriously don't need to turn the millet's volume past 9 when it starts at above 7. So I turned down the volume in itunes. This leads to bad results, the music just sounds bad (I am using foobar pass though and multiplugin). So I turned down the foobar volume, this is much better, but already I am starting to hear hiss at 11 oclock on the millet's volume knob. So 2 questions, is the millet going to have problem like this? and is there any way to mod the DAC-AH so I can tone down the gain of the output stage?

 

[sbelyo]

sounds like the gain is too high like you stated. The millett (if I remember correctly) has a high gain to begin with.

Try a resistor at least twice the value of your volume pot on each channel of the millett inputs

 

[Jahn]

bypass the output stage? this is what lan did for mine - not certain if the signal is quieter, but i definitely hear no hissing.

 

[n_maher]

The hiss is likely source related as I've never had a Millett hiss. Hum a bit with bad wiring yes, hiss no. Try it with a different source (CDP, DAP) if you can and see if the hiss is still there or try it with the Millett not connected to anything. Also, how does it sound if you run it comp -> Millett -> Headphones? Do you have to turn the amp up louder to get the same volume? If the Lite DAC does in fact provide some voltage gain (weird for a DAC IMO) you're running the signal through multiple gain stages which is less than ideal. Unfortunately the Millett's gain is set by the tubes and not adjustable and I wouldn't recommend adding resistance to the amp's output just yet. While it might "fix" the problem it'd be better to determine the problem's source first.

So more info is needed.

Nate

 

[dsavitsk]

Quote:

Originally Posted by n_maher If the Lite DAC does in fact provide some voltage gain (weird for a DAC IMO)

DAC chips, at least the ones worth listening to

, output current. This is then converted into a voltage at the I/V stage (I=current, V=voltage.) There are a few (a very few) DAC's that use a passive I/V (which is just a resistor, or a resistor and a transformer) but this usually (unless you parallel a lot of chips) leads to a very low output voltage. In general, most of them have either an opamp, a tube, or a discrete SS stage of somesort that adds gain to get the voltage up to line level. So, unless the Lite DAC uses a passive I/V, it almost certainly adds gain.

 

[n_maher]

Quote:

Originally Posted by dsavitsk DAC chips, at least the ones worth listening to , output current.

Good to know, I guess I meant mostly that I was surprised that it would have a lot of gain built in. I would have thought that most were designed with either unity gain or something very low. Does that not makes sense?

Nate

 

[ScubaSteve87]

I think I'll try PMing Lan and see what he did, but I think the problem is def that the gain issues. Just straight out of my ipod there is no real issues I can hear. I just think that the gain on the DAC-AH is really high and then again through the millet makes it even worse. I know I do have socketed opamps in the DAC-AH. Can I put in another ompamp that might result in less gain. I am not at home, but ill check whats in there now. I think it is also possible to put in OPA227's. on diyclub's website it lists the EL2044 OPAMP in there now. Anyone want to recommend a replacement?

 

[n_maher]

Quote:

Originally Posted by ScubaSteve87 Can I put in another ompamp that might result in less gain.

This is highly unlikely, granted I'm not the best person to answer this but I've never heard of a fixed gain opamp. Usually the gain is set by other components (resistors) in the circuit.

HTH,

Nate

 

[bg4533]

Quote:

Originally Posted by ScubaSteve87 I think I'll try PMing Lan and see what he did, but I think the problem is def that the gain issues. Just straight out of my ipod there is no real issues I can hear. I just think that the gain on the DAC-AH is really high and then again through the millet makes it even worse. I know I do have socketed opamps in the DAC-AH. Can I put in another ompamp that might result in less gain. I am not at home, but ill check whats in there now. I think it is also possible to put in OPA227's. on diyclub's website it lists the EL2044 OPAMP in there now. Anyone want to recommend a replacement?

My DAC-AH came with OPA602s soldered directly to the PCB? Are you sure your opamps are socketed.

The amount of gain is set by resistors, not the particular opamp. To change the gain you will need to change the resistors.

When I got my DAC-AH it hummed a bit too. Not horribly, but it was noticable on quiet passages. I modded my DAC-AH and the hum does not exist at any reasonable volume. I removed the opamps and took the output from the TDA1543s. I also inserted a steel shield between the transformer and PCB. I also replaced a lot of caps. All of the mods significantly improved the sound.

If you want to change the opamp in the DAC-AH almost any unity gain stable, single channel opamp should work. OPA627, OPA227, OPA132, AD8610, AD8065 are common choices.

 

[ScubaSteve87]

Quote:

Originally Posted by bg4533 My DAC-AH came with OPA602s soldered directly to the PCB? Are you sure your opamps are socketed. The amount of gain is set by resistors, not the particular opamp. To change the gain you will need to change the resistors. When I got my DAC-AH it hummed a bit too. Not horribly, but it was noticable on quiet passages. I modded my DAC-AH and the hum does not exist at any reasonable volume. I removed the opamps and took the output from the TDA1543s. I also inserted a steel shield between the transformer and PCB. I also replaced a lot of caps. All of the mods significantly improved the sound. If you want to change the opamp in the DAC-AH almost any unity gain stable, single channel opamp should work. OPA627, OPA227, OPA132, AD8610, AD8065 are common choices.

Yeah mine are sockted, the original owner ordered it from diyclub that way I think

 

[ScubaSteve87]

reviving this old thread. I just replaced my opa551's in my millet with diamond buffers, so could I throw these in as a replacement for the opamps in my DAC-ah? sorry for the newb circuit questions, but I don't know much about amp circuits at all

 

[dsavitsk]

Not sure what you would be hoping to gain from this? A couple of things you might try:

- Do as BG suggests and take the output right from the TDA1543. (You will be using a passive I/V. The TDA1543 is unique in being able to pass a usable level signal with just a resistor based passive I/V. To many of us, the sound is very improved by doing this.) To do this, you will need 5 resistors and 2 capacitors. The setup looks like this

Iout are pins 6 and 8, bias is pin 7. Assuming 8V on the DAC chip (pin 5 if you want to measure, and you do want to measure as the values will change with a different voltage), the first 2 resistors to ground are 2.7K, the next two to ground (the output resistors) are ~47K. The horizontal resistor (bias) is 1.4K. The two caps are (bi-polar) 2.2 to 4.7uF. Try the blackgate N series or Nichicon ES as both will work well here. If you have room for larger caps, look for some film ones.

If you don't have 8V on the DAC chip, or if you just want to understand what you are doing, here is a good explanation: http://www.diyaudio.com/forums/showt...ht=#post173483

Not only should this greatly improve the sound, but it will also slightly lower the output level. The quality of the resistors and caps makes a huge difference here, so choose carefully. I've had decent luck with PRP and Draloric. Less with the Dale and panasonic, so so with Holco, and excellent results with Riken and Kiwame. YMMV.


- A second option to lower the gain. If you remove the cathode bypass capacitors from the Millett (both the electrolytics and the film caps -- don't jumper the spots, just leave them empty) it will lower the gain by adding negative feedback. Mind you, I've not done this to a Millett so I have no idea what it will do to the sound, but the gain should go down.

 

[Thaddy]

ScubaSteve, did you ever fix the original problem? I plan on getting the DAC-Ah to use with my Millet and don't want to put more money into it just to make it listenable. I mean, I'll probably get it modded to some degree eventually, but I'd prefer to listen to it in stock form for a bit.