Line out - amp in level?

[JayH]

Modern separate high-end equipment tends to have the configuration of a preamp/line stage which has the input selectors, magnetic phono preamp, and volume control. This then connects to a power amplifier, which may or may not have a volume control.

Questions:

1. When building a power amplifier for use in such a scenario, is it customary to equip it with a volume control? I have seen amplifiers with and without.

2. If no volume control, is there a generally accepted standard input voltage for rated amplifier output? I'm primarily concerned with a hi-impedance unbalanced RCA setup, but if there is a standard for XLR inputs that would be nice to know as well.

[preiter]

Originally posted by JayH
1. When building a power amplifier for use in such a scenario, is it customary to equip it with a volume control? I have seen amplifiers with and without.[/B]

Entirely up to the builder. Sometime people modify the gain when they intend to use the amp as an integrated amp (with volume control). 33 is a typical gain for an integrated amp, 20 is more typical if you intend to use a pre-amp.

Originally posted by JayH
2. If no volume control, is there a generally accepted standard input voltage for rated amplifier output? I'm primarily concerned with a hi-impedance unbalanced RCA setup, but if there is a standard for XLR inputs that would be nice to know as well. [/B]

I believe the standard output for CD players and such is +/- 1V, but it can vary. Pre-amps often some gain, usually around 2.

[chrispenycate]

The standard professional level for balanced inputs is +4 dBu (0 dBu = 1mW into 600?, or 0.775 volts RMS) which makes it more or less a volt. However, overhead above this level can vary from +18 dB (eight times) to about +24 (sixteen times) Domestic levels are 10-12 dB below this, but potential peak levels vary wildly.
Comes the question - if you don't put a volume control on your amp, do you make it clip at FSD, the same time as your driving aparatus, at zero level, or some arbitary point between these two? There are arguments for each of these solutions. Evidently, if you're installing a volume control, you put in the higher sensitivity and attenuate to taste. (on the other hand, apart from fuses, I've had more problems with pots than any other component in power amps)
With a professional balanced input, the balancing stage (be it active or transformer) will generally be before the volume control, so it's important that it can accept maximum level with no audio degradation.

[AndrewT]

Hi,
in the old days, post valve gear, the power amp and integrated amp sensitivity tended to fall in the range 200mV to 500mV. More recently this has crept up to between 400mV to 1V, all rms or AC.

Part of the resason for the upward trend is the digital standard of 2V maximum output and the mistaken belief that the sensitivity should be approaching this value.

It turns out that most CDs are mastered to never exceed 2V output on peaks, but this demands that the average level is about 10db to 20db below maximum. -20db just happens to be 200mV, almost exactly the same as the old range of typical outputs from analogue sources. Some music will produce average output well below 200mV

The standard for professional gear is much higher to help attenuate the effects of interference due to the very long cables that abound in the commercial side of sound re-inforcement. There is a tendency for domestic gear to adopt some professional standards, or at least to move towards them, in the hope that buyers may consider equipment to be better made if it meets a professional standard. Ignore professional standards, most domestic equipment will distort/overload badly if their +18dbu/+24dbu signals were applied.

Stick within a sensitivity range of 200mV to 1V for a power amp, but ensure if you go towards the upper end you retain sufficient gain upstream to get adequate replay volume.

Rather than a volume control, how about a switched attenuator built into the power amp? levels of 0db,-6db and -12db will give adequate range to suit any normal (non professional) pre-amp output. if you adopted 0db=250mV, then -6db=500mV and -12db=1V sensitivity.

[troystg]

Dragging this old thread out from storage...

I am doing something similar and have a couple of questions.

I am taking the buffered tape outputs from a home receiver, summing them through 10K Ohm resistors and feeding the single channel into a Marchand XM-1 electronic cross-over. That in turn feeds a Crown amp.

The problem is that at full volume on the 10K pot the signal lights on the Crown just baaaarreely flicker.


I "assume" the 10K summing resistors along with the 10K log pot are attenuating the -10db signal too much to be useful on the Crown expecting +4db.

So I am going to bump the gain on the XM-1 to 20db, but I would like to get a tad bit more gain to ensure I can clip the crown and then set it's input controls accordingly.

http://www.marchandelec.com/ftp/xm1man.pdf

Any suggestions welcome.


EDIT: forgot the 10K pot after the summing resistors and the XM-1 module...

[dmills]

Why use 10K summing resistors? I would have thought 1K or even lower would be nearer the mark.

Assuming the crown is an old microtech or the like, they can be switched for 0.775V, 1.4V or a fixed 26db voltage gain. IIRC the controls are on the plug in PIP card.

One of my beefs with professional kit is that the power amps tend to be stupid sensitive, we have LMS systems (or graphics or desks or whatever) that manage +24dBu cleanly, why have the amplifier clipping at +4 or so?

I cannot see a good reason not to have the amps clip at +20dBu or so, given that all the gear intended for driving them will manage that level quite happily.

Regards, Dan.

[troystg]

Originally posted by dmills
1. Why use 10K summing resistors? I would have thought 1K or even lower would be nearer the mark.

2. Assuming the crown is an old microtech or the like, they can be switched for 0.775V, 1.4V or a fixed 26db voltage gain. IIRC the controls are on the plug in PIP card.

3. One of my beefs with professional kit is that the power amps tend to be stupid sensitive, we have LMS systems (or graphics or desks or whatever) that manage +24dBu cleanly, why have the amplifier clipping at +4 or so?

4. I cannot see a good reason not to have the amps clip at +20dBu or so, given that all the gear intended for driving them will manage that level quite happily.

Regards, Dan.

1. The standard I have always seen for summing 2 channels was 10K so I went with the standard.

2. It is an old CL1. Fixed 1.4Volt input unless factory set to .76 Volt.

http://www.crownaudio.com/pdf/legacy/136712.pdf

3. Crown is my only piece of pro gear so no comment on that.

4. Same as #3.

[AndrewT]

Hi,
the term "summing resistors" applies only to a summing opamp.
If the next stage is not a summing opamp then the resistors won't work properly.
If the next stage is a summing opamp then the resistor/s will already be in place.
You need to find the schematic for the next stage and post it here to allow the summing circuit to be designed.

[troystg]

Originally posted by AndrewT
....You need to find the schematic for the next stage and post it here to allow the summing circuit to be designed.....

Last page of this:

http://www.marchandelec.com/ftp/xm1man.pdf

[AndrewT]

have you fitted the two 10k resistors to the p1-2 input?