[fc911c]

Hi

I wanted to know what DC volts should be at Phone output? I am getting as much as 6.4 Mv at low volume and it decreases as volume is raised to just over 1mv at about 3/4 volume and under 1mv at full. I didn't see this in other amps I've built. Is this normal?

Thanks
Frank

 

[FallenAngel]

It's strange that volume affects it so much but 6mV is nothing to worry about.

 

[balou]

First and most important question: Which opamp do you use?
Second: Which gain?

Looks like you use an opamp with an a bit high input bias current, and the resistor plus your-favorite-listening-volume-pot resistor values dont seem to be optimally matched. [see tangentsoft.net, "working with cranky opamps" for details]

but 6mV arent that bad, its well below the often cited 20mV reasonable limit (this 20mV seem to be chosen rather low, so that absolutely no damage can be done over very long times)

 

[fc911c]

Hi thanks for the help

I am using 2132, 132 and 2 stacked 634 buffers. Resistors is standard setup. At normal listening volume its about 2-3mv.

I am using power supply at 24v

Btw way it sounds fantastic.crisp,clean and big sound stage with my px100's.

Frank

 

[fc911c]

I just thought of something that might make sence. I have 4 1000uf caps.

Could this becausing it, too much voltage in reserve? just an idea.

Frank

 

[FallenAngel]

You should be OK with 4000uF capacitance, it's a bit much, but who cares

Quick question, what "24V power supply" do you use?

 

[fc911c]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif You should be OK with 4000uF capacitance, it's a bit much, but who cares Quick question, what "24V power supply" do you use?

Yeah bigger is better, LOL

I have a Jameco 24v which puts ot 22.5v and a Steps PS ver 1.2, set to 24v.

Frank

 

[balou]

Considering that you use 2/132, this seems to be a bit strange. These opamps have an extremely low input bias current. Well, maybe that's just normal with a pimeta that the DC offset varies that 'much' (it's actually still well below the reasonable limit)

 

[fc911c]

Quote:

Originally Posted by balou /img/forum/go_quote.gif Considering that you use 2/132, this seems to be a bit strange. These opamps have an extremely low input bias current. Well, maybe that's just normal with a pimeta that the DC offset varies that 'much' (it's actually still well below the reasonable limit)

Thanks for your help

maybe someone who has the amp can weigh in here?

Thanks
Frank

 

[joe_cool]

Just a thought but the amp may have some DC at the inputs; this will cause the output DC to vary with volume control setting.

 

[fc911c]

Quote:

Originally Posted by joe_cool /img/forum/go_quote.gif Just a thought but the amp may have some DC at the inputs; this will cause the output DC to vary with volume control setting.

thanks I will check the ipod

Frank

 

[Ikrit42]

Wouldn't DC at the inputs cause the voltage to vary proportionally to the volume, rather than inversely? Higher volume on the pot = lower pot resistance, so less voltage dropped, right?
(Unless I'm being completely stupid about this... quite possible.)

 

[majkel]

The DC offset will always vary if the amp is DC-coupled to the volume potentiometer because then the potentiometer does the part of the offset nulling job.

 

[fc911c]

Quote:

Originally Posted by majkel /img/forum/go_quote.gif The DC offset will always vary if the amp is DC-coupled to the volume potentiometer because then the potentiometer does the part of the offset nulling job.

Hi

so your saying that this is the case with the Pimeta amp, that it's DC coupled?

Thanks
Frank

 

[Pars]

Quote:

Originally Posted by fc911c /img/forum/go_quote.gif Hi so your saying that this is the case with the Pimeta amp, that it's DC coupled? Thanks Frank

That is correct. C1 comes with a jumper trace shorting the C1 pads on the board. If you want to use input capacitors (C1, for AC coupling), this trace must be cut.

I believe that Tangent recommends DC coupling, unless you think you will be using sources which have excessive DC offset. When the amp is DC coupled, the input impedence to the non-inverting opamp input is set by R2 in parallel with the volume pot (approximately). As you can see, this impedence will vary depending upon the position of the volume pot (0 ohms to 50K ohms in parallel with 1M ohms). Since this will only exactly match the impedence at the inverting input of the opamp (set by R3-R6) at one point of the pot rotation, the output offset will vary. Read Tangent's "Working with Cranky Opamps" for more info.