[dewardh]

Tom said the hiss comes from the gain being too high. He recommended that I replace the R9 and R11 resistors with 10k-ohm resistors to lower the gain.

That will reduce the gain (of a 250x2) to between 9 and 10, and may reduce noise coming from a noisy source (if that was your problem).  It will not reduce noise generated in the amplifier itself (no input connected, or input grounded).  It is important to distinguish noise source when attempting noise reduction.  It also helps clarify issues if the amp model and configuration (bridged? gain? etc.) and speaker sensitivity and impedance (especially above 5-10kHz) are mentioned.

[6SN7]

For what it's worth, I'm using two 120x2s in bridged mode as mono blocs fed by an AMC tube preamp into my Maggie MG12s (84db, 4ohm), and if I select CD on my pre without actually playing a disc, I can turn the volume control fully clockwise (very scary) and I don't hear anything even if I put my ear right up to the speaker.  I should add that I had Tom reduce the gain on my amps to 26 from the stock 32.

Enclosures are nearing completion--takes longer when you have to build two of them!
Bob

[mintzar]

My speakers are 97dB sensitivity. The hum is only in the highs and it is the same hum I had with an older amplifier I had. The older amplifier actually had a gain knob on the back, as the gain was turned down the hum went away.

The amplifier I'm using is the 1000w kit -- way overkill but the 15" woofers like power. I just added a second power supply and second transformer. The amp is unbridged. One amp board is running the highs and one amp board is running the lows in my setup.

The noise floor of the amp itself is quite low. I have it plugged into an isolation transformer and a high-end power cord. The gain on my amp currently is 32dB. Tom was the person who recommended getting the 10k resistor. I imagine he recommended that resistor because it will bring the gain down to 26dB. 9-10 sounds extremely low.

[dewardh]

My speakers are 97dB sensitivity. The hum is only in the highs and it is the same hum I had with an older amplifier I had. The older amplifier actually had a gain knob on the back, as the gain was turned down the hum went away.

That suggests noise that is coming from your source, in which case reducing amp gain may help.  That is different from the case of others who report noise with no input connected.

The amplifier I'm using is the 1000w kit -- way overkill but the 15" woofers like power. I just added a second power supply and second transformer. The amp is unbridged. One amp board is running the highs and one amp board is running the lows in my setup.

I do not see the benefit of using only one amp on each board, but with 97 dB sensitivity speakers you have far more power than you need anyway . . . so 250 Watts (into 4 Ohm) should be plenty.

The noise floor of the amp itself is quite low. I have it plugged into an isolation transformer and a high-end power cord. The gain on my amp currently is 32dB. Tom was the person who recommended getting the 10k resistor. I imagine he recommended that resistor because it will bring the gain down to 26dB. 9-10 sounds extremely low.

The gaine of the 250x2 modules is set by Rf/Rin.  Rf is 120k (in the reference design), and Rin is R11 (10k after the change) plus R12 (which is 3k on the boards I have and on the reference design boards.  These part numbers are for the classDaudio board non-inverted channel . . . the part numbers in the IR reference design are different.

Good luck with your project . . .

[zapper7]

dewardh,

Thanks for the explanation. It does make sense.
I am the one who has no input while still getting hiss. I just hooked a pair of Silverlines that are 92 db, and the hiss is almost gone. So my current 99db speakers are allowing the hiss through.
What can I do? I have the boards bridged to get the 500w, should I run the highs with an un-bridged board running one channel at 125w (I have an active xover with gains) to lower the hiss, or would it? I may try anyway. I may get other speaker too.

[mintzar]

Dewardh,

Let me correct something, I am running one amp board for the highs (running both channels on that board) and one amp board for the lows (running both channels). LR high and LR Low. I have an active XO and so I am biamping.

There is no noise coming from my source. It's just the interface between a high sensitivity speaker and a high gain amplifier. It's a hiss and not a hum and happens even when the source is unplugged, just like with the old amps. There is no hiss in my woofers, just the tweeter. From my experience, the hiss will be fixed with a lower gain amp.

[DougSmith]

500W with 99db speakers (are these horns)?  Unless I'm missing something you will have permanent ear damage in less than a minute!  Try them unbridged.

[mintzar]

They ARE unbridged, gents. My DAC is low gain as well so they aren't blaringly loud, nor do I play music loudly.

I'm running Emerald Physics, which use pro drivers. The tweeters are high-sensitivity compression drivers.

[zapper7]

I took the tweeter off the bridged mode and into 1 side of the stereo mode, the hiss remains the same.

I guess the speakers are too efficient for this type of amp to expect there to be no hiss. Like I said above, the 89-93 db speakers had hiss to maybe 2' from the speaker, which is better.

Tom mentioned something about adjusting the amp for high efficiency speakers, but never got back to me as to what that is or if I/ he needs to do it.

 

[roymail]

I'm thinking that gain reduction down to 26db may be your best bet.  From what I've read, Tom does that frequently for those using higher efficiency speakers. 

[dewardh]

I'm thinking that gain reduction down to 26db may be your best bet.

That would be a fairly "normal" gain for consumer amps, so it's a reasonably safe starting point.  IR seems to have chosen gain in the reference amps so that the same input will produce rated output in all the variants, on the assumption that amplifier power would be chosen appropriate to need.  ClassDaudio seems to have copied that in their initial designs.  The result, though, is unnecessarily high amp gain (on the higher power amps especially) when used in “consumer” applications, leading in many cases to excessive amplification of source noise.  Changing to a uniform gain across the line more nearly matches normal “consumer industry” practices.

But ultimately best results will be had when the amplifier is matched, both in power and gain, to the source, the loudspeakers, and the desired acoustic output.  As DIYers building our own dedicated systems we know the peak output level of our source, the sensitivity of our speakers, and our target peak acoustic levels.  From that it is easy to compute amplifier power needed, and the amplifier gain needed to reach that power (this kind of signal management is called “gain staging” in the pro-audio world).  Too much power, or too much gain, usually introduces noise problems, and sometimes an increase in small-signal distortion.

As an example:  a Seas W22EX driver has a sensitivity of about 88 dB.  60 Watts will produce an acoustic output of about 105 dB (a common design target) from it, and an amplifier gain of around 24 will reach that level with full output from common consumer sources.  The amp runs out of power, the speaker runs out of excursion (at low frequency, depending on the box), and the source runs out of output all at the same time . . . maximizing performance of all.
 

[atroder]

Could this gain issue be addressed by adding a rotary gain dial to the amp? This would preclude having to swap resistors to dial in the correct gain. Wish I knew what I was talking about

[Nick77]

Tom has included adjustable gain on the new prototype boards soon to be released for newcomers.

[jtwrace]

Tom has included adjustable gain on the new prototype boards soon to be released for newcomers.

now that's cool.

[dewardh]

Tom has included adjustable gain on the new prototype boards soon to be released for newcomers.

That would be very nice . . . and all it would take even on the existing boards would be 10k trimpots in place of the two input resistors.

[zapper7]

dewardh,

Can you elaborate on the pots and let me know which resistors would be replaced and where to get the pots, I like the idea of being able to dial the amp in, like the pro amps I have running the subs in my room.

Thanks

[ebag4]

If you change out the resistors and use pots won't yo need to know where and what to measure to insure you have them set correctly or would you adjust them before placing them in the ckt and hope you don't bump them?

Thanks,
Ed

[zapper7]

Mintzar,

I do not see a R9 or 10, On my boards it seems to be 60-61

[AmpDesigner333]

Just a few simple hints...

1. Make the power supply as large as possible (big electrolytic caps especially) and as high voltage as specs allow
2. Keep signal path connections as short as possible and use shielded wires, ground the shields
3. Practice good grounding techniques --- beginners should search the net for "star ground"

Regarding #1 above, be sure to account for varying mains voltages when using linear supplies.  Use FAT wire.  Good luck once again.

[Mariusz]