How to go from RCA to XLR?

Hi Rww -- I'm having difficulty envisioning how that would work at all. What would the output configuration be, in terms of how the speakers are hooked up? And what would generate the inverted input signal?

Regards,
-- Al

Although unfortunately there are variations, the most common xlr pin connection arrangement is:

Pin 2 -- hot (the positive waveform if differential [balanced] signals are being sent from xlr outputs)
Pin 3 -- cold (the negative or inverse waveform if differential signals are being sent from xlr outputs)
Pin 1 -- ground

If that is the configuration of your monoblock inputs, the center pin of the rca connector at the other end should be wired to pin 2, and the rca shell (ground) at the other end should be wired to pins 1 and 3.

To verify the pinouts used on the xlr's on the amps, you could either try to obtain a schematic, or perhaps some Googling would lead you to the answer, or else use a multimeter to check for continuity between pin 1 and signal ground on the amp (probably the chassis).

Verifying the ground pin is key. Reversing hot and cold (usually pins 2 and 3) will reverse absolute phase but will not hurt anything. The sonic effects of absolute phase reversal would be subtle, and would be correctable by changing plus and minus connections to the speakers, as well as by having correspondingly reversed connections in the xlr to rca cable.

Regards,
-- Al

I'd suspect so, but I'm not sure that would necessarily be true with all equipment. Try it and see. If you get continuity to one of the xlr pins, you've verified that that pin is ground. If you don't get continuity to any pins, then that approach won't provide you with an answer. Same goes for measuring to chassis.

By "continuity" I mean something very close to 0 ohms (or at most say 1 or 2 ohms), as measured on an ohmmeter, not just an indication from a buzzer or other rough continuity checker which could indicate continuity even if a significant resistance is present.

Also, when you measure it would be prudent to have all signal cables from other pieces of equipment disconnected from the amp, so that your measurement is not confused by any possible paths to ground through the other equipment.

Regards,
-- Al

With designs that I am familiar with, feeding a balanced differential amplifier input with a single-ended signal will only mean that you have to turn the volume control on the preamp up a bit higher, by 6 db. It will not affect the maximum power output of the amp, only the volume control setting on the preamp that is required to reach that maximum output.

I suspect that the person you spoke to at Aragon doesn't know what he is talking about. Or else their design is very unusual in some respect.

A transformer such as Rwwear suggests will get you back most of the 6db, if that matters, but a quality transformer is likely to cost significant $, and even a good one may still have subtle sonic effects.

Regards,
-- Al

Atmasphere, isn't losing 6db of gain the same as losing half the output of the amp or more?

No. That's the basic point that both Atmasphere and I were making, which the person at Aragon apparently doesn't understand. Losing 6db of gain just means that the volume control on the preamp has to be turned up a little bit higher (6 db), to drive the power amplifier to the same maximum output power level that it would otherwise have reached at a 6 db lower setting of the volume control.

There is absolutely no difference either way in the amplifier's maximum power output. The only difference is in the volume control setting at which that maximum happens to be reached, which is a completely inconsequential difference, except perhaps in an extremely rare situation in which the preamp and power amp are severely mismatched in terms of gains and levels.

Regards,
-- Al

Hi Rwwear,

First, I would not assume that the monoblock amps are designed internally as bridged stereo amps, unless you know that to be true for the specific model.

But in order to answer your question, let's assume they are. I'm pretty certain that the design of a mono amp which is internally a bridged stereo amp, and has a balanced differential input, would NOT have one of its two amplifier sections driven off of the positive-going input line, and the other off of the negative-going input line, which I think is what you are envisioning.

If it were done that way, that would defeat the fundamental noise reduction advantage of having balanced interconnect cables and interfaces. As you probably realize, by feeding both the positive and negative signal inputs into a differential receiver device, common mode noise that can be expected to be present equally on both lines gets cancelled.

I believe that what would be done in the design of a mono amplifier that was internally bridged and had differential inputs is something conceptually similar to the following diagram. Note that the second diagram, in fact, shows a single ended input and a differential output, from the same circuit that can also be used to receive a differential input:

http://www.edn.com/contents/images/84302f4.pdf

The op amp that is used in this case (the same concepts would apply to an amplifier stage made up of discrete transistors) receives either a balanced differential input, or a single-ended input referenced to ground, then amplifies the difference between the two inputs, and then in either case outputs an out-of-phase (balanced differential) pair of signals that in turn would then branch off to the two amplifier sections as you envisioned.

So both output amplifier sections would still be driven, and therefore the result would be no reduction in output power capability, just a 6db reduction in gain as I and Atmasphere indicated. Perhaps Atmasphere or someone else can confirm that I am envisioning the design correctly.

Thanks for your good question!

Regards,
-- Al

Rww -- I get your drift; touche! :)

But the op should note in the reference that he can get a great deal of noise rejection just with the cabling arrangement that Whitlock describes (his diagram labelled "correct" on page 29), without the expense and possible sonic signature of a transformer.

And as I indicated earlier, even if he just uses the adapter as originally envisioned (Whitlock's diagram labelled "wrong" on page 29), he would have noise performance comparable to a fully single-ended arrangement, which may prove to be satisfactory anyway.

Regards,
-- Al

Rww -- I'm not certain I understand your question, but I think you are asking why shouldn't all monoblock amps, at least the better ones that have balanced topologies, be designed as bridged amps.

I'm probably not the best person to give you a comprehensive answer on that, because although I am an experienced EE my professional background is primarily digital, not analog, and is unrelated to audio. But the basic point to bridging is, as I'm sure you realize, to increase output power capability. But that comes at the cost of the ability to drive low impedances, as you alluded to, and I'd imagine at the cost of a number of other conceivable distortion mechanisms. Off the top of my head that would include difficulty matching the two amplifier sections precisely in terms of many different parameters (gain, linearity, phase and frequency response), delay offsets between the inverted-polarity path and the non-inverted polarity path, etc.

And whatever the reasons may be, I think the conventional anecdotal wisdom is that more often than not bridged stereo amps often just don't sound as good as when those same amps are used in normal stereo mode.

Let me know if I misunderstood your question.

Regards,
-- Al

MBL whom offers some of their most costly designs as either stereo single or a mono pair that simply uses the XLR input to make them balanced mono only.

Rww -- I took a look at the MBL 9007 via the link you provided. Yes, it pretty obviously is designed to work in a bridged configuration when used as a mono amp, since it can also be used as a stereo amp. Just goes to prove the old saw about the design approach that is chosen being less important than how well the particular approach is implemented. Not sure why they didn't provide balanced inputs for stereo use, though, unless they just felt that from a marketing standpoint those who would buy just one amp and use it in stereo mode would be less likely to be driving it with a balanced preamp than those buying two amps for monoblock use.

Such a design cannot be used in single ended mode as a mono amp.

Not by going in through the single-ended inputs, but I would think you could do just what we have been discussing, namely going in through the xlr connector with an adapter that grounds one of the two polarities.

As Shadorne states, that would of course sacrifice the noise and ground loop-related advantages that the true balanced interface would provide. But I think it is important for the OP to keep in mind that the results he can expect in those respects would not differ dramatically from those that a comparable fully single-ended (unbalanced) system would provide.

Regards,
-- Al

Rww -- yes, but they still could have provided balanced xlr inputs for each stereo channel, and then fed those inputs into a pair of differential receivers with single-ended outputs. The single-ended outputs could then have been fed into the same path that the single-ended rca inputs in the actual design are fed into. That would add the benefits of a balanced interface and cabling, without changing the internal architecture significantly (other than the addition of a few components).

they are using the two separate channels combined to make the amp balanced .... That's the reason you probably should not use an RCA adapter to convert an amp to single ended in this type of design.

Well, ok, I don't have sufficient familiarity with this type of design to comment on this.

Regards,
-- Al

See pages 28 and 29 of the following paper for a description of how to feed a single-ended signal into a balanced input without major degradion of noise performance. My thanks to Shadorne for calling this paper to my attention.

http://www.jensen-transformers.com/an/generic%20seminar.pdf

Regards,
-- Al

That sounds ok to me at the output side. The speaker would be connected between the two positive output terminals of the amp (I think, although I'm not certain offhand, that you would want to use the 16 ohm taps if you have 8 ohm speakers), and the two negative output terminals of the amp would be left unconnected (they are tied together and to signal ground internally).

On the input side, I think you are saying that you would take your xlr source (transformer or preamp), and construct a cable that ran xlr pin 2 to the center pin of an rca plug, and xlr pin 1 to the ground of that rca plug, and ran xlr pin 3 to the center pin of another rca plug and xlr pin 1 to the ground of that second rca plug. The two rca plugs would then plug into the amp's two rca inputs. Correct?

Or are you saying that you would replace the rca inputs on the amp with an xlr connector that you would install, with pin 1 wired to amp ground, pin 2 to the single-ended signal path of one channel (wherever the center pin of the rca had been wired to), and pin 3 to the single-ended signal path of the other channel?

I think that everything would function either way, but I'm not sure that you would gain all that much in terms of noise rejection, especially if you don't replace the rca input connectors. Balanced inputs reject noise that is present equally on both polarities, but with the two polarities being physically branched off I'm not sure that noise pickup would be really equal. You will gain a lot of power, though.

Regards,
-- Al

What the manual describes is not differential and is not balanced. It just converts the amplifier to mono by paralleling up the channels, resulting in a single-ended amp with twice the power.

-- Al

Yes, well as far as I can tell I think you'll succeed in both respects, if you install an xlr jack connected as I described, and connect the speaker between the + outputs as I described and not as the manual describes.

Good luck with it!

-- Al

A couple of further thoughts.

It will be important to make the two channels as identical as possible, meaning identical tubes, identical bias settings, etc. Otherwise the positive and negative parts of the waveforms will be treated asymmetrically, which is equivalent to a distortion.

Also, I take back what I said about possibly using the 16 ohm taps. I think that would result in too much current flowing. Perhaps someone else could suggest which taps would be best to use.

-- Al

Noslop -- Thanks for the witty presentation of your empirical findings! :)

But I don't think that they necessarily support the conclusion that the rca-to-xlr conversion is what is responsible for the huge volume difference that you heard between the two configurations.

Even if the power amplifier architecture corresponds to what Rwwear had suggested, which is the worst case in terms of the possible effects of the conversion on the amp's power output, there would only be a 6db loss in maximum output power. That represents a 75% reduction in maximum amp output power, which in subjective terms is not anywhere near the difference you describe, imo.

Putting db losses into perspective, it is commonly said that a 10db loss (which is a loss of 90% of the maximum output power) corresponds to a subjective perception of "half as loud." What you are describing sounds like a greater loss than even that, given that with the balanced preamp you had the volume control at less than 1/2 scale, while with the unbalanced one you had it at max.

So something else is going on, obviously related to the extra variable of having two different preamps in the comparison. Perhaps their gains are different, or perhaps something in the complex set of settings of the Stage One is confusing things.

I think that a better test would be to compare balanced vs. unbalanced outputs of the Aurum, with the unbalanced Aurum output going into the power amp through the adapter.

To summarize what has been discussed as to the different possible effects of the adapter on the power amplifier output, depending on its architecture:

If the power amp architecture is as I and Atmasphere envisioned, there would be no sacrifice in maximum output power by feeding it single ended through the adapter; it would just be necessary to raise the volume control 6db higher to reach that maximum output power (compared to balanced drive).

If the power amp architecture is as Rwwear envisioned, then the maximum output power of the amplifier (the point at which it would clip) would be reduced by 6db (compared to balanced drive), and that reduced clipping point could not be overcome by the volume control.

I think that the fact that you were apparently unable to clip the power amplifier when driving it with the adapter, even with the volume control at max, reinforces my view that your findings so far are inconclusive.

Regards,
-- Al

If as I think the amps are bridged, what happens to the channel that isn't available when using the RCA adapters? It would be half of the amp's power being sent to ground.

That was what I was addressing in my previous post when I said:

If the power amp architecture is as Rwwear envisioned, then the maximum output power of the amplifier (the point at which it would clip) would be reduced by 6db (compared to balanced drive), and that reduced clipping point could not be overcome by the volume control.

Actually, 3/4 of the amp's maximum power output would be lost (a 6db reduction), not 1/2 (a 3db reduction). One side of the amp would swing to the output voltage it is supposed to. The other side of the amp, instead of swinging to the same voltage but with the opposite polarity (minus instead of plus, or vice versa), would be at 0. So the net voltage difference across the speaker that is connected between the two outputs would be half of what it is supposed to be. Half the voltage = 1/4 of the power = -6db.

If an attempt were made to overcome that loss by turning up the volume control, then on loud peaks of the music, that would cause the amp to have to output more power than that reduced amount, the sound would clip/breakup/distort. Noslop did not report that he heard any distortion, just that the volume was lower, which tells me that something else was going on, such as his unbalanced preamp having less gain than his balanced preamp.

It's important to keep in mind that reduced power capability does not in itself mean reduced volume (although there would be reduced volume as well, by 6db). A moderate loss in volume, such as 6db, can be overcome by turning up the volume control a bit. What a loss of output power capability does is to reduce the volume level at which clipping/breakup/distortion occurs.

Regards,
-- Al

I think his last paragraph confirms that you can.

Regards,
-- Al

Yes, I was just about to post the following, when I saw Atmasphere's last response:

I don't see why I would need an input transformer Al.

You don't. I think Atmasphere is addressing the fact that an unmodified Citation II has single-ended inputs. In your case, as we discussed earlier, you would be substituting an xlr for the single-ended input jacks, wiring the two signal carrying pins of the xlr to wherever in the amp the center pins of the existing rca jacks go, and wiring the ground pin of the xlr to amp ground. And you'll be feeding it with a balanced source. I think that accomplishes what Atmasphere was referring to.

Regards,

-- Al