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Crown D-75a as Monoblocks - Polarity Question?


I have had two of these amps for ages, but I have been using just one in stereo so far which has been a reliable workhouse of an amp on my desktop.

Today at last, I have changed the internal jumpers to bridge them into mono and followed instructions in the manual for both to wire the speakers correctly and use only Channel 1 on each amp.

The good news is

1) They seem to be working using Channel 1 only as an input and the Channel 1 volume controls only on the front panel

2) They sound stronger and seem to have more bass

3) On simple stereo left and right tests, they seem to be working properly

The bad news however, is that there is a slightly weird, headachey, sounds not quite out of phase muddiness to the center image - even though I can hear stereo separation between left and right.

This goes away when I press the MONO switch on my DAC / preamp, although then the sound is clearly in mono.

Googling around, there is some discussion of polarity, and custom wires (?) to be used when bridging these amps into mono??

I have been experimenting with audio for > 30 years now, but I have absolutely no idea what anyone is talking about and have pasted some text below if anyone can help.

Thank you and best regards,



Greg Taylor wrote:
>
> Which leads me to a more general question. If the normal phase
> difference between the two channels is 180 degrees does it make sense to
> swap pins 2 and 3 on one of the inputs, or rather change the polarity of
> the speaker connection on one channel, in 'stereo' mode to avoid LF
> cancellation effects from the two speakers?

I'm not sure what you're talking about here? Are you talking about bridged
mono?

The whole idea of bridged mono is that you have both outputs referenced
to ground. You put a signal of opposite polarity into each input, so
now you have the two outputs swinging in opposite directions. This is
like putting two amplifiers in series with one another... you get twice
the output voltage but you don't get any more current. You can't swap the
polarity on the output side because the grounds are already tied together.
If you floated one output with a transformer you could, but that gets silly.

If you are talking about the easiest way to swap polarity on one channel
of a stereo system, it really doesn't make any difference whether you swap
on the input or the output. If you're using banana plugs for speakers, it
is easier to swap the speaker side. If you're using Speakon connectors,
it's probably easier to swap the input side.
--scott
cwlondon
4 responses Add your response
cwlondon OP
1,210 posts
 
Since posting, I did a bit of Googling and had a memory of liner notes on the original Dave Grusin Discovered Again LP which led me to experiment last night by reversing the polarity on BOTH speakers.

This seems to have improved the situation significantly, although some comments on the internet refute that this should have any effect?

More center focus now and as stated in the original post, definitely more power and more bass - both welcome.

Still, however, a slightly wider and more diffuse center image - although very clear left and right separation!

Could it be possible I am just getting used to much better channel separation as a result of using two amps instead of one?

Still dont know why I would need to do any wiring other than hooking them up using Channel 1 only, as stated in the instruction manual.

This was also a cinch when I was using a pair of Behringer A500s on the desktop, but fit finish and controls nicer on the Crowns.

Thanks for any technical ideas.
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almarg
9,670 posts
 
Hi CW,

Nice to see you back here.

The quote from "Greg Taylor" is addressing how to correct an issue of having the two channels out of phase with respect to each other, i.e., an issue of "relative phase." It does NOT sound like you are facing that issue, because having the two channels out of phase with respect to each other would result, when a mono signal is fed into both channels, in an image that is vague, diffuse, and hard to localize, and in weak bass.

The first major paragraph in the response from "Scott" is pretty much just describing how bridged mono works. The second paragraph answers Greg's question, saying that from a technical (as opposed to a practical) standpoint it doesn't matter which of two methods is used to correct a difference in relative polarity between the two channels (in a situation where balanced XLR interconnects are being used, which would allow the polarity of a channel to be inverted by interchanging pins 2 and 3 on the XLR connector at one end of the cable; alternatively of course the + and - connections to a speaker could be swapped).

What your second post indicates you've just done is to invert "absolute polarity," which as you realize changes the polarity of both speakers at once, keeping them in phase with each other. The audible significance of doing that is controversial, and is recording dependent IMO. To the extent that it may make any difference at all, it can be expected to make the most difference on recordings that have been engineered using "purist" techniques, meaning a minimal number of microphones (e.g., just 2 or 3), with minimal or no subsequent mixing, compression, processing, etc.

Beyond that, I have no particular thoughts about the phasiness you have described hearing.

Best regards,
-- Al
cwlondon OP
1,210 posts
 
Al,

Thank you so much and good to hear from you. I managed to get through to a tech at Crown yesterday, where his advice was also beyond the scope of my technical background.

But to paraphrase, he said that a bridged amp addressed the waveform a bit differently than an amp operating in stereo? And therefore, that imaging might naturally be affected, and thus a bridged set up might not be best for me after I assured him I was an old school, neurotic audiophile.

I remembered the advice from the liner notes of Dave Grusin which was indeed a "purist" direct to disc recording from Sheffield Labs. I was fascinated as a kid to change the polarity on my Infinity Qa's but dont recall hearing a significant difference. Another poster on the web also suggested that any improvements were controversial.

In any case, I am now confident that everything is wired correctly, and at least I am enjoying more power and more control in the bass.

As an aside, the Crown guy sounded very experienced and although he described the D-75a as a classic, said that he was also an audiophile and their new D amps including the XLS 2000 could hold their own against any Levinson (?!)

If so, that might be a new giant killer for $500?

Happy Thanksgiving,
almarg's avatar
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almarg
9,670 posts
 
... he said that a bridged amp addressed the waveform a bit differently than an amp operating in stereo? And therefore, that imaging might naturally be affected, and thus a bridged set up might not be best for me after I assured him I was an old school, neurotic audiophile.
Yes, bridged mode can certainly be expected to sound somewhat different than stereo mode. But the most predictable situation in which it "might not be best," for many bridged designs, would be if the speakers have impedances that are significantly less than 8 ohms, especially in the bass region (where lots of energy is typically required).

The reason for that is that in bridged mono mode a given speaker impedance will be "seen" by the output stage of each of the two amplifier channels as 1/2 of that impedance value. That is because for a given output voltage that is produced by each of the two output stages twice as much current will have to be supplied by each stage as in stereo configuration, since the "other side" of the load impedance (i.e., the speaker terminal that is connected to the other output stage) is being driven to an equal and opposite voltage by that other output stage. The voltage that is applied across the speaker terminals thereby being doubled.

So as is often the case for bridgeable amps the D-75a does not have a maximum power rating for 4 ohm loads when in bridged mode. A 4 ohm load would be "seen" as 2 ohms in bridged mode, which the amp presumably would not be able to handle with good results.

In any event, although I don't know what speakers you are driving with these amps, the fact that you are perceiving stronger and better bass would seem to bode well.

Happy Thanksgiving to you and yours as well!

Best regards,
-- Al
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