[Joe Presto]

Quote:

Originally Posted by acvtre /img/forum/go_quote.gif Maybe it's better if you use an impedance adaptor for the headphones.

Quote:

Originally Posted by john57 /img/forum/go_quote.gif No from what I understand the user want is a way to lower the gain using a higher resistance pot for the volume control should do the trick. Changing the impedance could effect the SQ.

Can I switch out the ALPS pot for a higher resistance pot? if so, any advice on what value to switch it with?

Edit: I originally planned on putting the pot between the amp and the headphones, however what John suggests would be far more elegant.

 

[tempzx]

Quote:

I got one of these back in October and I never really messed with it (ac/dc adapter arrived dead and had to wait another 2 weeks before I could even use it). For some reason there's a LOUD buzzing noise when the knob is anywhere passed 9 o'clock. Tried different outlets and messed with the pots etc.. nothing got rid of it. Anyone know what would be causing this buzzing noise? Could it be the tube? I've also heard faint clicking/popping noises every now and then coming from the amp itself. Anything would be helpful.

Edit: Ended up finding a bad solder joint while checking it out in front of my computer screen.. Figures I can't find the soldering iron when I need it.

 

[oohms]

Quote:

Originally Posted by Joe Presto /img/forum/go_quote.gif Can I switch out the ALPS pot for a higher resistance pot? if so, any advice on what value to switch it with? Edit: I originally planned on putting the pot between the amp and the headphones, however what John suggests would be far more elegant.

Or you could put a 50-100k resistor in series between the line inputs and the volume pot pins they connect to
(Or make a line input cable with these resistors in it)

 

[Joe Presto]

Quote:

Originally Posted by oohms /img/forum/go_quote.gif Or you could put a 50-100k resistor in series between the line inputs and the volume pot pins they connect to (Or make a line input cable with these resistors in it)

You are suggesting that I add resistance to the amp input, so between source and amp, or between amp input and volume pot? I am confused now, and really looking for the easiest way to get my Grado's to a listenable level.

 

[CPL593H]

Quote:

Originally Posted by Joe Presto /img/forum/go_quote.gif You are suggesting that I add resistance to the amp input, so between source and amp, or between amp input and volume pot? I am confused now, and really looking for the easiest way to get my Grado's to a listenable level.

I don't think that you can add resistance to the input to reduce the gain. The standard method is called an L-Pad. (no relation to he iPad). The L-pad goes on the output.

Here's an article on lPads. You should be able to make one with a couple resistors rather than buying one for $20.

Uneeda Audio - Build your own attenuator pads

 

[Joe Presto]

Quote:

Originally Posted by CPL593H /img/forum/go_quote.gif I don't think that you can add resistance to the input to reduce the gain. The standard method is called an L-Pad. (no relation to he iPad). The L-pad goes on the output. Here's an article on lPads. You should be able to make one with a couple resistors rather than buying one for $20. Uneeda Audio - Build your own attenuator pads

Thanks for the link, I'll plough through the article tonight and see if I can build it!

 

[CPL593H]

Quote:

Originally Posted by Joe Presto /img/forum/go_quote.gif Thanks for the link, I'll plough through the article tonight and see if I can build it!

Here's a schematic that the Bravo amp was based on. Component values may be different.

That article I posted is too difficult to deal with. Here's an L-pad calculator. http://amps.zugster.net/articles/attenuation

if you input the impedance of your Grados (SR325is are 32 ohms) and 10 dB of attenuation (about half as loud) in the calculator it will give you some values for an L-pad that will be a starting point. good luck

 

[Joe Presto]

Quote:

Originally Posted by CPL593H /img/forum/go_quote.gif Here's a schematic that the Bravo amp was based on. Component values may be different. That article I posted is too difficult to deal with. Here's an L-pad calculator. Adam's Amplifiers: Attenuation if you input the impedance of your Grados (SR325is are 32 ohms) and 10 dB of attenuation (about half as loud) in the calculator it will give you some values for an L-pad that will be a starting point. good luck

Cheers CPL593H, I did some digging and found the calculator, I was trying to find out how much attenuation I needed, so thanks for your suggestion. I'll build a rudimentary one first and make one out of proper components if it works out.

I looked up that schematic earlier, but couldn't make heads nor tails of it, I was trying to determine if the volume pot is the last component the signal goes through before the headphone out, in which case might it be possible to switch the volume pot?

 

[dafoomie]

Would you guys say that these are unsuitable for Grados out of the box?

 

[CPL593H]

Quote:

Originally Posted by Joe Presto /img/forum/go_quote.gif Cheers CPL593H, I did some digging and found the calculator, I was trying to find out how much attenuation I needed, so thanks for your suggestion. I'll build a rudimentary one first and make one out of proper components if it works out. I looked up that schematic earlier, but couldn't make heads nor tails of it, I was trying to determine if the volume pot is the last component the signal goes through before the headphone out, in which case might it be possible to switch the volume pot?

if you look at the schematic on the left (not the actual measured data one) you see Input on the left side and Output on the right side.

The input goes into the volume control pot (yellow 10K-20K) then connects to the grid (g) on the tube.

The output on the right comes out of the IRF610 MOSFET output transistor through the 1000uF capacitor to the 47 ohm resistor. The LM317 is a voltage regulator for the tube heater.

That strange looking stuff in the upper left (couple overlapping circles) is a transistor current source.

Finally there is a yellow resistor with an arrow through it 2.2K, that's the cathode bias pot. This is the pot you adjust to get 16-17v on the plate (labeled a on the tube)

This is one of the two channels of the amp.

 

[oohms]

Correct me if i'm wrong, but wouldn't a resistive attenuator at the output change the sound characteristics? (since the headphones are an inductive load)

 

[Joe Presto]

Quote:

Originally Posted by dafoomie /img/forum/go_quote.gif Would you guys say that these are unsuitable for Grados out of the box?

I would say so. The channels on mine are imbalanced at low volume, once I turn it up far enough to have them balance, I find that the volume is just a bit above comforatable listening level, tried with 60, MS-1i and 325i. YMMV

 

[CPL593H]

Quote:

Originally Posted by oohms /img/forum/go_quote.gif Correct me if i'm wrong, but wouldn't a resistive attenuator at the output change the sound characteristics? (since the headphones are an inductive load)

you're right. how much it colors the sound depends on too many things to calculate, the headphones are reactive and nonlinear as is the amp. Just have to try it and see how it works. Professional audio uses L-Pads, T-Pads all the time.

 

[CPL593H]

Quote:

Originally Posted by upstateguy /img/forum/go_quote.gif 8610s are the default opamps so it's probably a good choice. Why don't you put it in a cardboard box for the time being and fire it up, it sounds great. USG

Well a small USPS priority mail box made a perfect size case for the M^3 and it sounds amazing. Put the OPA627s in it. Now waiting for the replacement K702's. Thanks for the box suggestion.

 

[oohms]

The volume control essentially forms an L pad at the input, so by adding a resistor there would preserve the sound characteristics of the amp better.. but the setup would be more prone to noise, than if you attenuate at the output.