Matching input gain for 2 amps

[Luvchampagne]

Question on gain differences between XLR and RCA outs. Trying to match gain between 2 amps.

I have a Krell HTS 7.1 as my preamp and 3 rca input only amps, MC7270s.
1 amp is used for mid/highs speaker input and 2nd set is used bridged mono for bass input. Please put aside the biamp issues.

Krell has both XLR and RCA output. It also has a Zone 2 RCA output.

First assumption is that the gain with Amp 1 vs. Bridged pair of Amp 2 and Amp 3 are roughly the same. Correct?

What is the gain when you output from BOTH the RCA and XLR simultaneously? I would guess that the gain is double in the XLR output over the RCA output. Correct? Not mentioned in the Krell manual.
However, if I use an XLR to RCA cable, then the gain drops but still higher than RCA output gain?

Also, if you are outputting to 3 rca input only amplifiers, would you recommend -

XLR out Xlr/rca cable for 1 amp and RCA out to the other Amps. Gains are probably mismatched.

Or XLR out to a XLR splitter/rca cable to all amps? Higher gain and potentially less interference and noise.

Or RCA out to rca splitter to the amps? Less gain due to RCA splitter and potentially more noise.

Or RCA out to 1 amp and Zone 2 RCA out to other amps.

Thank you

 

[Kal Rubinson]

First assumption is that the gain with Amp 1 vs. Bridged pair of Amp 2 and Amp 3 are roughly the same. Correct?

Unlikely but possible. Need to check the specs in the amp manual.

What is the gain when you output from BOTH the RCA and XLR simultaneously? I would guess that the gain is double in the XLR output over the RCA output. Correct? Not mentioned in the Krell manual.
However, if I use an XLR to RCA cable, then the gain drops but still higher than RCA output gain?

Your presumptions are reasonable but, without specifications or actual measurements, they are not assured.

Also, if you are outputting to 3 rca input only amplifiers, would you recommend -

XLR out Xlr/rca cable for 1 amp and RCA out to the other Amps. Gains are probably mismatched.

Or XLR out to a XLR splitter/rca cable to all amps? Higher gain and potentially less interference and noise.

Or RCA out to rca splitter to the amps? Less gain due to RCA splitter and potentially more noise.

Or RCA out to 1 amp and Zone 2 RCA out to other amps.

Rinse and repeat. You need the specs.

FWIW, the voltage loss due to the passive splitter is dependent on the current output capabilities of the preamp, the output impedance of the preamp and the input impedance of the amps.

 

[Sokel]

Not a good idea to short the outputs with XLR>RCA.
Check with the manufacturer if possible without leaving blue smoke out of it.

Bridging amps usually gets their gain higher,I usually see 6dB,depending the amp.
If I was you I would measure the output of the amps with a DMM while all connected through RCA and use attenuators if needed.

 

[Luvchampagne]

Unlikely but possible. Need to check the specs in the amp manual.

Thank you very much for your reply.

Unfortunately Mcintosh manual doesn't have that information. Krell manual also doesn't have that spec and Krell is impossible to reach.
I will keep looking.

Your presumptions are reasonable but, without specifications or actual measurements, they are not assured.

Rinse and repeat. You need the specs.

FWIW, the voltage loss due to the passive splitter is dependent on the current output capabilities of the preamp, the output impedance of the preamp and the input impedance of the amps.

 

[Kal Rubinson]

Unfortunately Mcintosh manual doesn't have that information. Krell manual also doesn't have that spec and Krell is impossible to reach.

Krell is under reorganization consquent to the death of the major owner but you can probably contact Mac.

 

[DVDdoug]

If you are uncertain it might be best to use an SPL meter (or an SPL meter app for your cell phone) and measure the output just-below, and just-above the crossover frequencies to make sure everything is matched.

...Normally I don't trust SPL apps because the mic in phones isn't calibrated. But in this case you just want to make sure you're getting the same measurement from each driver (at almost the same frequency) so it doesn't matter if the absolute SPL reading is wrong.

(Audacity can create test-tones at the frequencies of your choice.)

 

[DonH56]

A brief search did not yield output levels for the Krell preamp (though I only looked through a few of many, the factory specs listed do not provide output levels). It is common for XLR outputs to have twice the voltage swing (thus 6 dB higher output) compared to RCA. If you use an XLR to RCA cable that leaves one of the balanced signals open then you'll have half the signal. I do not know if the RCA and XLR outputs are isolated (separate output buffers).

McIntosh has the MC7270 manual online at https://www.mcintoshlabs.com/-/medi...revision=9f6626bb-eb8f-4970-bb50-f2c707082134 It says there is a switch to select between 0.75 V and 2.5 V input sensitivity (10.5 dB difference), and then there are gain controls. In bridged mode the gain is thus higher (ideally 4x power so 6 dB higher gain in bridged mode). The input impedance is 20k ohms so three in parallel would be 6.7k, a fairly low load (two in parallel is 10k).

You can buy a passive 6 dB in-line RCA attenuator to feed the bridged amp. https://www.amazon.com/Harrison-Lab...id=1739818854&sprefix=6+dB+RCA,aps,170&sr=8-1

Note if you do not have a crossover before the amplifiers, all will have the same input signal and all will need the same frequency and power range, so you may want to consider adding a crossover if you are bi-amping (I do not know if the Krell has one built in). There is a link in my signature to articles and one discusses bi-amping with and without a crossover, so-called "passive" bi-amping.