Passive preamp and biamp

[zigo3]

Hello
I want make a simple passive preamp to attenuate my 2 poweramp for a biamp configuration.
The first powamp, that we call "A", is a 20 wpc, 20 khom input impedance; the second, that we call "B", 200 wpc, 25 kohm input impedance.
The source has a 75 ohm output impedance.

The simplest solution is drive the 2 powamps with 2 different attenuators, but all times that I change the volume output I must re-balance the 2 powamps inputs, a very boring procedure if you do it several times a day.

I was thinking to another solution: use a potentiometer, that we call "X" to drive both the powamps and a second one, that we call "Y" to balance only one powamp, the one who needs to be attenuate to arrive to the level of the other one (I suppose in my case the B). In this way I make the balancement only once and all times that I change the volume I must turn only one potentiometer.

To do it I was thinking to use for the X potentiometer a resistor attenuator 5K inpedance (or 10k) and like Y a alps (or similar) better if linear, for fine balancing regulation, and low impedance, 1k or so.

What I don't know is: if I put 2 potentiometers, 5k + 1k, what will be the total output impedance?
What do you think about this solution?

In a second time, when the configuration is "stable", I was thinking to substitute the second potentiometer simply with a couple of resistors, to have (I suppose) better quality output. Do you think is a good solution, or it happens that changing the volume I must remake fine balancement corrections?

Thankyou very much in advance.
rob

[AndrewT]

Hi,
can your 75r source drive a 5k pot?

Wire the the 5k pot output to both amps. The maximum output impedance of the 5kpot+75r source is ~ 1270r. This can drive short low capacitance cables.

If the balance between high and low is not quite right then adjust the higher gain amplifier down slightly, but be careful you don't reduce the phase margin which will make the amplifier less stable.

[zigo3]

By first thanks a lot for your reply.

Originally posted by AndrewT
[B]Hi,
can your 75r source drive a 5k pot?

I don't know, I have read around and I have seen that usually for sources about this impedance and powamps 30/40 Kohm are used 5/10k attenuators. This is the reason because I supposed to use it. Maybe a lower imped. attenuator should be better?

Wire the the 5k pot output to both amps. The maximum output impedance of the 5kpot+75r source is ~ 1270r. This can drive short low capacitance cables.

How can I measure the cable capacitance? Low capacitance means little diameter? 1 meter max is short enough?

If the balance between high and low is not quite right then adjust the higher gain amplifier down slightly, but be careful you don't reduce the phase margin which will make the amplifier less stable.

I will make some tests for that. I can use a 1k potentiometer without problems for that?

thanks again
rob

[AndrewT]

Originally posted by zigo3
How can I measure the cable capacitance? Low capacitance means little diameter? 1 meter max is short enough?


I will make some tests for that. I can use a 1k potentiometer without problems for that?

small diameter and high dielectric constant = high capacitance.
Usually foam or cell dielectric (air instead of solid) is lower capacitance and usually large diameter is lower capacitance.

1m of 100pF/m cable will add 100pF to whatever is fitted inside the receiver. It may be OK.

75r source is very unlikely to be happy driving a 1k load.
even a 2k load could be a problem.

try not to add a second pot. It will load the first pot and ruin the balance between channels and ruin the log law your ears expect to hear.
Instead find out whether the power amps and speakers have the same sensitivity and try to balance those by adjusting the gain if possible.
Ideally a bi-amped system uses the same gain in the amplifiers and the same sensitivity of speakers.

if you feel you must cascade potentiometers then the 10:1 rule still applies.

Rin=20k requires a pot ~5k
The 5k pot requires a 500r pot infront of it.
The 500r pot requires the source to drive the 500r load. No!

Do you realise that buffering the sources will solve all these impedance problems?

[zigo3]

Wow...thankyou very much Andrew!!!

Now I begin see more clear.
...I think I will look around for buffers: I didn't realize how important is the source output impedance. I supposed that less stages between source and power amps was the best solution, but they are limits.

[zigo3]

mmm...after I read something I think that ANYWAY I will try the passive: even using a buffer I must buy a pot...then let's try it alone before.
I have read that there is the 100 times rule too, but having a 75 Ohms suorce output and a 25000 Ohms powamp input anyway I must look for a compromise: very short and low capacitance cables and a 10k pot (if I will use stepped resitors: I think is the lower impedance disposable around) or a 5k (if i will use motorized alps or similar). For lower impedance pot I found very few models in stereo.
..and then I will compare it with the active pre that I can use at the moment. If it works well (like a lot of people affirms) obviously I prefer this simpler solution.

About biamping I will make some tests: I bought a TRMS voltmeter (9,99$) to measure the powamps output at the same volume level, I will measure the attenuation needed to eventually balance the powamps output and then I will put a resistor between the pot and the powamp who has an higher gain: this will take problems?

[adason]

I want make a simple passive preamp to attenuate my 2 poweramp for a biamp configuration.

I think you need a buffer, otherwise sound will suffer...

Another option not mentioned here before that could work best if you do not want to use buffer would be to feed the fist A amp, take output from this amp (besides going into A speakers), feed the input of second amp. I have done it before and it works better than all passive split option.

Ideally a bi-amped system uses the same gain in the amplifiers and the same sensitivity of speakers.

Nah, we don't live in ideal world. Most of the time main advantage of biamping is to have speakers with different efficiency and being able to compensate with different volume setting on each amp.

[zigo3]

Originally posted by adason

Another option not mentioned here before that could work best if you do not want to use buffer would be to feed the fist A amp, take output from this amp (besides going into A speakers), feed the input of second amp. I have done it before and it works better than all passive split option.

..god... really? It don't explodes?
Maybe is a good idea put a potentiometer even between powamps A and B?
Anyway it is a fantastic solution: simple and clean.

Nah, we don't live in ideal world. Most of the time main advantage of biamping is to have speakers with different efficiency and being able to compensate with different volume setting on each amp.

eh yes! this is one of the main reasons because I biamp too.
Everything in HIFI is a compromise...

[AndrewT]

Hi,
bi-amping uses the standard crossover inside a standard speaker.
Splitting the input terminals and feeding each from a power amp demands that the gain of the power amps be identical.

If you want or need to take advantage of different sensitivities of the driver, then that means removing the standard crossover.
Now, you are into the realms of designing an active crossover. All the bi-amping rules go out the window.

[zigo3]

Originally posted by AndrewT
Hi,
bi-amping uses the standard crossover inside a standard speaker.

Are you making an etymological point about the term "biamplification"?
You can make bi-tri-etc. amping using different drivers in different loudspeakers if you want, using crossover or not. In a 3 way loudspeaker I divided bass driver from mid and high (together) and I biamped using the crossover working on the volume to balance them. You can feed different drivers directly too using different amps, or decide to feed directly one driver in a loudspeaker bypassing the crossover and use the crossover to feed the other drivers of the same loudspeaker (closed or open baffle).

How do you call it?