I intend to build an amplifier based on 3e Audio's TPA3255 board. I'd like to include a volume control, but I don't need an input selector or tone controls, so hopefully I can just add a (passive) volume control.
In this article I read that there should be high ratio between source output impedance and amp input impedance, something like 100:1 or more. Given that the input impedance of the TPA3255 board in 10kΩ, the output impedance of my source (a Topping E30 DAC) shouldn't exceed 100Ω. Unfortunately, the output impedance of this DAC is unknown (Topping doesn't mention it in the specs).
Would it be recommended to just add a stereo potentiometer between the DAC and the TPA3255 board? If so, given that the output impedance of the DAC is unknown, what resistance would you recommend for this potentiometer? And I assume I need the logarithmic type?
In this article I read that there should be high ratio between source output impedance and amp input impedance, something like 100:1 or more. Given that the input impedance of the TPA3255 board in 10kΩ, the output impedance of my source (a Topping E30 DAC) shouldn't exceed 100Ω. Unfortunately, the output impedance of this DAC is unknown (Topping doesn't mention it in the specs).
Would it be recommended to just add a stereo potentiometer between the DAC and the TPA3255 board? If so, given that the output impedance of the DAC is unknown, what resistance would you recommend for this potentiometer? And I assume I need the logarithmic type?
Even if it is unknown, it should be a few 100 ohms max. A 10k volume control will be fine. The 1:100 is overrated, it will make sure that you loose not more than 1% of your signal level (which you can compensate for by turning the volume up a very small amount), but even 1:10 is acceptable.
And yes it should be a log taper pot, our hearing is logarithmic in snsitivity and a log control gives a nice smooth linear impression.
Jan
And yes it should be a log taper pot, our hearing is logarithmic in snsitivity and a log control gives a nice smooth linear impression.
Jan
If you are keen on preventing any kind of conflict regardless of source though, it's easy to build a very very cheap buffer circuit with a cheap op-amp, which will multiply the current delivered by your DAC multiple times and as thus, provide as much current as the amp wants - all the while having a very high input impedance of its own allowing flawless volume control use. 🙂
(theory behind this says that less current flow inherently means less current to be affected by the impedance of the potentiometer)
Potentiometers do add a certain level of distortion to the spectrum of frequencies, because the variable resistance together with cable capacitance forms a variable filter - neat! The effect is only measurable, and sometimes audible for some select people. Yes, chances are greater that your TPA amp has greater non-linearity in the spectrum. 🙂
(theory behind this says that less current flow inherently means less current to be affected by the impedance of the potentiometer)
| But... "just a pot" should do fine unless you're looking for extreme perfection in both high and low volume situations. |
Potentiometers do add a certain level of distortion to the spectrum of frequencies, because the variable resistance together with cable capacitance forms a variable filter - neat! The effect is only measurable, and sometimes audible for some select people. Yes, chances are greater that your TPA amp has greater non-linearity in the spectrum. 🙂
It is not often that you see a post that is mostly technically OK but completely useless to the OP. But the above pulled it off.
Jan
Jan
Thanks for all your insights and advice!
I've decided to get a good 10k logarithmic pot (something like an Alps RK27), and hook things up for testing first, before I hack up the amp enclosure.
I've decided to get a good 10k logarithmic pot (something like an Alps RK27), and hook things up for testing first, before I hack up the amp enclosure.
It is better to take one of the DACT potentiometers because there is less chance of channel imbalance.
That will work, although in this case a linear pot would be best, because the load is also 10k.
You may have to turn up the Alps control rather high to compensate for the 10k load.
Any solid state source can drive 10k easily.
All good pot perform worst at -6dB.
With such good amp, direct dac to amp input (balance out to in) will get the best result.
With such good amp, direct dac to amp input (balance out to in) will get the best result.
I would think using a pot more than twice cost of the amp board seems kind of over the top. Though I'd rather not use it, my DAC can also provide level control, which I'd rather use that spend >€200 on a pot.
Good to know, I'll get the linear type instead.
I don't understand what you mean with "perform wordt at -6dB", but my DAC, the Topping E30, does not have a balanced output.
I understand, I've been looking into boards like these, but that would also require a dedicated AC power supply, adding cost and complexity.
At -6dB, the input will see 5k ohm (assume uses 10k pot) impedance.
This 5k ohm will couple (Johnson) noise into the amp.
It's better to get a balance out dac and directly input to the TPA3255.
If use single ended, more noise will be produce with the inverting op amp (see TI EVK).
Thanks, I understand (to some extend at least). I assume noise would result in a hiss, but do you expect this hiss to be audible at reasonable volume levels? Would the use of an active preamp also be a solution (instead of going balanced)?
I think I'll try the unbalanced route first with just a pot between the DAC and the amp. If results are not satisfactory I can always switch to an active preamp or balanced DAC.
Should not be audible 1m away. Just that this amp is really good but the implementation will reduce slight performance.
When using pot, be careful of the impedance change caused by the op amp negative input.