Input Sensitivity of McIntosh Power Amplifiers

[m.tonetti]

I was thinking about the different input sensitivity of McIntosh power amplifiers.

The technical characteristics of interest, for the analysis, are the following:

MC275
Sensitivity Input: 1.2V Unbalanced, 2.5V Balanced
Impedance Input: 90kOhms Unbalanced, 180kOhms Balanced

MC252
Sensitivity Input: 1.6V Unbalanced, 3.2V Balanced
Impedance Input: 10kOhms Unbalanced, 20kOhms Balanced

MC402
Sensitivity Input: 2.0V Unbalanced, 4.0V Balanced
Impedance Input: 10kOhms Unbalanced, 20kOhms Balanced

MC501
Sensitivity Input: 2.1V Unbalanced, 4.2V Balanced
Impedance Input: 10kOhms Unbalanced, 20kOhms Balanced

MC2301 (never tested)
Sensitivity Input: 1.7V Unbalanced, 3.4V Balanced
Impedance Input: 47kOhms

MC1.2KW (never tested)
Sensitivity Input: 2.5V Unbalanced, 5.0V Balanced
Impedance Input: 10kOhms

Considering the different input sensitivity of power amplifiers the problem is how get the attenuation value of the input signal for the more sensitive power amplifier, usually tube technology, for Mid/High section.

The relating formula for tensions ratios is the following:

Att(dB) = 20 * Log10(V1/V2)

Where V1 and V2 are the input sensitivity for the two power amplifiers. It isn't important the role of V1 and V2, the change is only the sign of the logarithm.
This formula is valid only if the input and output voltages are applied to the same impedance.

In my system I'm using in passive bi-amp mode, the following combinations: MC501 (Low) and MC275-MC252-MC402 (Mid/High).
For example in the case of coupled MC501/MC252 we have a value of 2.3dB. That's the reason I have never noticed big differences between the two sections!
The same McIntosh, in the user manual of MC2301 - p.15, proposes the passive bi-amp using the MC2301 for section Mid/High and through the out connector of the same MC2301, an MC1.2KW for the Bass section.
In this case the value of the attenuation calculated for the coupled MC1.2KW/MC2301 is 3.3dB.

However in my case (MC501/MC252) using a simple, but it seems valid, DBX 223XL (or Bryston 10B STD Bal) for section Mid/High, I could cut at 200Hz (since the frequency of crossing Low/Mid crossover passive for B&W 802N speakers is 350Hz) and set an attenuation of 2.3dB.

If I will buy an MC2301, the coupled MC501/MC2301 in the balance mode, I will have a value of 1.8dB, which is even lower.

Any suggestions or feedback?

I apologize for my poor english!

[Mctwins]

Hi
I think that McIntosh works like this:

If you have a McIntosh preamp with meters and in balanced mode (xlr) that reads 0dB you will have full rated power to your amplifier that reads 0 dB. If my memory is correct the McIntosh amps has also around 2.1 dB headroom.

But it depends what the powerrating is for the seperated bass repective mid/high and also what impencance is.

The thing is that the high-end speakers never say's what the powerrating is for the respective enclousures, bass or the M/T. It is very difficult to bi-amp a system without this data.

I don't notice any difference regardings the input sensitivity in my system-
Thanks

[m.tonetti]

In this case the value of the attenuation calculated for the coupled MC252/MC275 is 2.1dB. Probably that's the reason you have never noticed differences between the two sections!

[Mctwins]

Probably....

I have DBX PA+ and when I run autolevel feature the gain is +1dB in the lowpass and -1dB in the high pass. Changing the gain on the DBX 223XL as you are suggesting and setting the attenuation to 2,3 dB is not the same, it is very difficult to see the changes and to balanced the level between low and high pass.

DBX PA+ has as microphone to the jobb right. There is something called a GAIN STRUCTURE and it's totally different. You can not do this with McIntosh amps bacause it has fixed input sensitivity. That's how I see it.
Thanks

[m.tonetti]

Why 2.3dB?

I had seen from your signature (now disappeared!) that you have a double couple of MC252/MC275 and DBX223XL then I calculated 2.1dB!

With your speakers Von Schweikert VR-4SR you use two DBX223XL?
Because DBX haven't the separate high and low pass frequency adjustments and VR-4SR cannot eliminate the internal crossover.

In the owner's manual of VR-4SR the problems related the different input sensitivity is highlighted.

Below an extract:

"BI-AMPING THE VR-4SR Mk2
If the tightest bass is desired, use a solid state amplifier on the woofer modules. If you value “image float” and “liquid”-sounding midrange/treble response, use a tube amplifier on the M/T’s.
You will not need an outboard electronic crossover, since the crossovers in the speakers will still continue to work.
Although it is true that louder treble output can be obtained by high-pass filtering the tube amp, the loss of transparency is usually not worth it.
The clean volume obtainable without a high-pass crossover will be usually satisfying for anyone but a metal head.
(Note: if you are a metal head, welcome to high-end audio!) You will need a preamp that has twin outputs, but if you have only one set, use high quality Y-jacks available from your high-end dealer.
Balancing of the volume levels when using amps with different input sensitivities can be accomplished by outboard level controls available from your dealer at very modest cost.
If the amplifiers have input sensitivities which are fairly close, even if they are different brands, there will be no need for level controls.
VERTICAL BI-AMPING:
If two stereo amplifiers of the same brand and model are available, try using this method of wiring.
One channel will drive the woofer module, while the other channel of the same amplifier will drive the upper M/T’s.
HORIZONTAL BI-AMPING:
If two different stereo amps are available, one amp can drive the woofers and the other amp, perhaps of lower power, can drive the M/T’s.
This works well with large solid state amps driving the woofers, with a smaller tube amp driving the upper end.
We have heard of systems with 200 watts per channel on the woofers, with 20-30 watts per channel of tube power driving the M/T sections.
This may seem like an imbalance, but since the most power is needed to drive the woofers, it may work very well!"

Thanks in advance.

[Mctwins]

I was thinking of this in your first post:

However in my case (MC501/MC252) using a simple, but it seems valid, DBX 223XL (or Bryston 10B STD Bal) for section Mid/High, I could cut at 200Hz (since the frequency of crossing Low/Mid crossover passive for B&W 802N speakers is 350Hz) and set an attenuation of 2.3dB.

I am well familiar with the owner manual with VR4SR. But I don't agree with this about running with tube amp on M/T because it is not good for the tube amp to run without an active x-over and it is better to remove those freq it is not working( in my case removing freq 20-150 Hz) and the tube amp will work better and more efficiently and with less distortion. I added (didin't have too) one more to the solid amps as well but it isin't so much important as with the tube amp. Talk to Ron C he will explain it better than me.

I have been running in mono blocks to each modules on VR4SR.

[m.tonetti]

Originally Posted by Mctwins

... I don't agree with this about running with tube amp on M/T because it is not good for the tube amp to run without an active x-over and it is better to remove those freq it is not working( in my case removing freq 20-150 Hz) and the tube amp will work better and more efficiently and with less distortion. I added (didin't have too) one more to the solid amps as well but it isin't so much important as with the tube amp. ...

I agree with you.