Amplifier hiss with Denon x3700h preout

[BullBuchanan]

I recently picked up two mono block ampliers from an apparently defunct USA company called Portal Audio. These models are called Paladin and we're manufactured in 2006.

I'd like to sell them, but I want to make sure they're in good working order before I do. The problem is I'm hearing an audible (from the listening position) hiss coming from the tweeters of my speakers (two sets tested). The cables aren't long enough to reach both

Here's an example of the hiss sound: https://photos.app.goo.gl/uVDcbq97UrvfsCrL9

Setup:
Denon AVR x3700h
Portal Audio Paladin Monoblocks
Speakers: Tekton Double Impact, Von schweikert VR-4SR MK2

I have the left amp and recover plugged into a surge protector and the right amp plugged into a different surge protector (same make) in a different outlet.

I just bought new rca cables to try to rule that out. I did get some audible improvement with the new cables. The hiss is now unstable and even sound where before there were crackles and the right channel was louder. Now they are the same volume and texture. The hiss does not increase/decrease with volume changes on the AVR.

When I run the AVR as an amp, it's dead silent.

Because the noise is consistent now across channels, I see two likely possibilities. Either the noise is inherent to the preout of my AVR or it's inherent to the design of the amplifiers.

I can't think of how I'd verify either as I don't have another preamp to test the amplifiers, and the amp manufacturer is no longer in business and has sparse detail on the wen outside of a stereophole review from 2006.

And other tools I could buy or ways I could root cause the noise?

 

[respice finem]

If the noise is still there after turning off the AVR, it's coming from the "Paladin" amps I would say.

 

[BullBuchanan]

If the noise is still there after turning off the AVR, it's coming from the "Paladin" amps I would say.

Thank you. I guess that should have been obvious.

When I turn off the AVR the hiss drops off substantially to the point it can't be heard at the Listening position. There's a faint sound still coming from the tweeter if I put my head up to it, but that seems less concerning and it stable across the two amps.


So I guess it's a bad/poorly designed pre-out section on the denon?

 

[respice finem]

Or too high amp gain maybe, or the wrong input impedance... The amps experts might be able to answer this.

 

[TankTop]

The Denon pre amp outputs test below human hearing threshold. I second that it’s likely your output voltage gain setting but combined with your amplifiers.

 

[BullBuchanan]

The Denon pre amp outputs test below human hearing threshold. I second that it’s likely your output voltage gain setting but combined with your amplifiers.

By "output voltage gain setting" are you referring to the voltage gain of the amp?

The specs list the input impedance at a switchable 10k/100k, though with single ended (which I'm running) the 100k turns into 50k. Switching this had no effect
The specs list the voltage gain of the amp at 28db, but stereophile measure it at 35.7 when running single ended.
Input sensitivity was not listed in the spec or the review.

Portal Audio Paladin monoblock power amplifier Measurements | Stereophile.com

Sidebar 3: Measurements I preconditioned one of the Portal Paladin monoblocks by running it for 60 minutes at one-third the measured clipping power into 8 ohms. With the amplifier cold, the measured THD+noise was 0.06%; with it hot, this changed only very slightly, to 0.064%. At the end of the...

www.stereophile.com

In my limited understand of voltage gain, it's the increase on top of the voltage output of the amp pre-out. In the case of the Denon, that's 1.4V, but shouldn't that voltage scale with the volume setting on the pre-amp? The volume of the noise doesn't increase or decrease with volume changes. The only changes are when turning the Denon off.

 

[staticV3]

In my limited understand of voltage gain, it's the increase on top of the voltage output of the amp pre-out. In the case of the Denon, that's 1.4V, but shouldn't that voltage scale with the volume setting on the pre-amp? The volume of the noise doesn't increase or decrease with volume changes. The only changes are when turning the Denon off.

Changing the Denon Pre out volume should not change the hiss, as that is controlled digitally so the noise level is likely constant.

In the end, it sounds like a classic case of gain mismatch.

Put an analog Preamp between Denon and Paladin, something like the Topping L30II, set the Denon to Full output and control volume using the L30II, and the hiss should disappear.

Ditto if you just use a source device with lower output voltage + lower inherent noise. Something like the Apple headphone adapter + 3.5mm to 2x RCA adapter.

 

[BullBuchanan]

Changing the Denon Pre out volume should not change the hiss, as that is controlled digitally so the noise level is likely constant.

In the end, it sounds like a classic case of gain mismatch.

Put an analog Preamp between Denon and Paladin, something like the Topping L30II, set the Denon to Full output and control volume using the L30II, and the hiss should disappear.

Ditto if you just use a source device with lower output voltage + lower inherent noise. Something like the Apple headphone adapter + 3.5mm to 2x RCA adapter.

Your comment about lower output voltage makes perfect sense to me.

I don't quite follow why an analog pre-amp is also a solution though. How does digital vs analog impact this scenario?

 

[staticV3]

I don't quite follow why an analog pre-amp is also a solution though. How does digital vs analog impact this scenario?

Digital volume control: signal amplitude changes, noise stays constant (->full SNR only at max output)

Analog volume control: noise drops alongside the signal

 

[BullBuchanan]

Digital volume control: signal amplitude changes, noise stays constant (->full SNR only at max output)

Analog volume control: noise drops alongside the signal

That's very interesting and illuminating. So does that mean that the SNR/SINAD of a digitally controlled amp is only meaningful at max volume?

 

[voodooless]

Changing the Denon Pre out volume should not change the hiss, as that is controlled digitally so the noise level is likely constant.

Not quite. The control might be digital, but the actual chip doing it is actually an analog device, so it works just like any opamp.

 

[BullBuchanan]

Not quite. The control might be digital, but the actual chip doing it is actually an analog device, so it works just like any opamp.

Then is there a different explanation for the noise not increasing or decreasing with the volume of the receiver?

 

[voodooless]

Then is there a different explanation for the noise not increasing or decreasing with the volume of the receiver?

So, given no difference in volume, we either look at the amp, or at the combination.

To figure out what is going on, it would be good to describe the noise. It is more of a buzz, or is it a soft wideband noise?

Can the noise be lowered if you touch some bare metal parts of the amp, like one of the RCA terminals?

 

[BullBuchanan]

So, given no difference in volume, we either look at the amp, or at the combination.

To figure out what is going on, it would be good to describe the noise. It is more of a buzz, or is it a soft wideband noise?

Can the noise be lowered if you touch some bare metal parts of the amp, like one of the RCA terminals?

It's a hiss: https://photos.app.goo.gl/uVDcbq97UrvfsCrL9

 

[voodooless]

Okay, so wideband noise. What if you short the amp input? Is it quite then?

 

[BullBuchanan]

Okay, so wideband noise. What if you short the amp input? Is it quite then?

Meaning remove the RCA input? If so, it gets much, much louder.

 

[voodooless]

Meaning remove the RCA input? If so, it gets much, much louder.

Remove it, but then short the middle and ground pin. If you leave the cable in, and do this on the other end, that might be easier. What if you touch either ground or signal pin on the other end of the cable?

 

[BullBuchanan]

Remove it, but then short the middle and ground pin. If you leave the cable in, and do this on the other end, that might be easier. What if you touch either ground or signal pin on the other end of the cable?

• I left the RCA plugged into the input of the amp and removed the end from the output of the receiver.
  • This caused a very loud hiss.
• I then put a multi-meter testing probe touching the ring and pin of the side I unplugged.
  • the hiss went away leaving seemingly perfect silence

 

[DVDdoug]

The Denon pre amp outputs test below human hearing threshold.

We can't know that. We don't know the SPL level. It depends on the gain of the amplifier, the sensitivity of the speakers, how close you are to the speakers, and audibility also depends on acoustic noise in the room.

 

[voodooless]

the hiss went away leaving seemingly perfect silence

Well, then it seems the issue might be the higher-than-spec gain, if we can believe the Stereophile measurements.

So the 10/100k input impedance switch doesn't make a difference?

You might just try to run the amp with XLR input to use the lower gain, just get an RCA to XLR cable (or make a decent one yourself):

D

'RCA to XLR Cable ("pseudo-balanced") to buy ?'

Feb 1, 2023

just a short question here:

I want to buy (not DIY) RCA to XLR (male) cables in the ("pseudo-balanced") fashion that Hypex and others recommend (see picture below).

I want to buy ready made / 'off the shelf' cables (of several different lenght) that are of very good* "mechanical" quality,
*by this I mean good materials and well done workmanship, ("should be pleasing to my fingers and eyes") -> I do not mean any cable-woodoo silver rhodium ultra high resolution blah blah

Any quick recommendations from your experience (a link is sufficient).




drawing from Hypex Datasheet:...

• Dirk_67
• Replies: 52
• Forum: General Audio Discussions

• 

Also: did you configure the x3700h to disconnect the amps for the pre-outs?

Connecting an external power amplifier AVR-X3700H

manuals.denon.com