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Deciding on Permanent Amp + Utilizing Attenuators Between Tube Pre-Amp and Amplifier

Ngen52

Ngen52

New Member
Hi guys,

I recently picked up a pair of Shure A15AS XLR attenuators (set to -15db) to run between my tube pre and solid state power amp.

The amp is a loaner from a good friend that I may purchase but it’s much more ~typical audiophile~ than anything I’ve ever considered before.

My speakers (JBL 4722N) are incredibly sensitive and with the higher power of the amp I’ve been using (Classe CA2100), both buffer and tube mode on my Schiit Freya+ were producing a mild hiss that did not get louder with volume increase but was still audible from the listening position.

I presume this is because of gain stacking / 104db sensitivity speakers since the noise was non-existent with my tried-and-true Dynaco ST-70.

Upon adding the attenuation, the noise was eliminated entirely but it also seemed to shift the tonal balance (from tubes operating more linearly instead of less volume knob being used maybe?)

Is anything about this gain/attenuation scenario sub-optimal?

Are there any obvious oversights I’m missing when implementing this amp in my signal chain?

Current Signal Chain:

Room: 12x14x8’

Treatment: (2-4) 12x48x3” broadband panels on side and rear walls, floor-to-ceiling 24” wide super-chunk style bass traps.

Speakers: JBL 4722N Two-Way ScreenArray® Cinema Loudspeakers

Equalizer: Schiit Lokius

Subwoofer: MartinLogan Depth I

Amplifiers: Classe CA2100

Pre-Amp: Schiit Freya+

Pre-Amp Tubes: NOS Sylvania 6SN7GT on output stage, Tung Sol 6SN7GTB on input stage

DAC: Schiit Gungir Multibit

Phono Pre-Amplifier: Schiit Skoll (Phasemation T550 when running MC cartridges)

Turntable (Motor): Denon DP3000 housed in high mass plinth

Turntable (tonearm): Dynavector DV-505

Turntable (cartridge): Audio-Technica AT-VM740xML VMx Phono Cartridge
 
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I poked around a bit and here's a thread that has some talk that's maybe a bit over my head. It may be helpful: https://gearspace.com/board/so-much...y-does-shure-a15as-change-my-snare-sound.html

Also FWIW whenever I was looking into this I always read that in-line attenuators should be wirewound to minimize impact on the sound. Shure doesn't say how these are constructed but the switching capability alone would make me pause.
 
Jfaustus said:
I poked around a bit and here's a thread that has some talk that's maybe a bit over my head. It may be helpful: https://gearspace.com/board/so-much...y-does-shure-a15as-change-my-snare-sound.html

Also FWIW whenever I was looking into this I always read that in-line attenuators should be wirewound to minimize impact on the sound. Shure doesn't say how these are constructed but the switching capability alone would make me pause.
Click to expand...
I believe they’re a T-pad / H-pad style circuit made from metal-film or carbon film resistors!

I’ll check that out tho!! Thank u!
 
The resistor based attenuator added impedance between your pre and power amps and that changed the sound.

You could use a transformer or autoformer based attenuator which wouldn't add impedance but would cost much more.
 
Ngen52 said:
I recently picked up a pair of Shure A15AS XLR attenuators (set to -15db) to run between my tube pre and solid state power amp.

Is anything about this gain/attenuation scenario sub-optimal?
Click to expand...
Those attenuators are designed for microphones. The input impedance is 1k according to Shure's specs.
www.shure.com

A15AS User Guide - Shure

Online user guide for Shure A15AS switchable in-line attenuator.
www.shure.com www.shure.com
I can't imagine that your tube preamp is happy driving a 1k load.
 
it wouldn't, the amp is just too sensitive. If there are no level controls on the amp, an attenuator between the preamp and power amp is the sensible way to deal with that issue. I don't know those specific attenuators so they may not be the right ones to use.
 
Ngen52 said:
Hi guys,

I recently picked up a pair of Shure A15AS XLR attenuators (set to -15db) to run between my tube pre and solid state power amp.

The amp is a loaner from a good friend that I may purchase but it’s much more ~typical audiophile~ than anything I’ve ever considered before.

My speakers (JBL 4722N) are incredibly sensitive and with the higher power of the amp I’ve been using (Classe CA2100), both buffer and tube mode on my Schiit Freya+ were producing a mild hiss that did not get louder with volume increase but was still audible from the listening position.

I presume this is because of gain stacking / 104db sensitivity speakers since the noise was non-existent with my tried-and-true Dynaco ST-70.

Upon adding the attenuation, the noise was eliminated entirely but it also seemed to shift the tonal balance (from tubes operating more linearly instead of less volume knob being used maybe?)

Is anything about this gain/attenuation scenario sub-optimal?

Are there any obvious oversights I’m missing when implementing this amp in my signal chain?

Current Signal Chain:

Room: 12x14x8’

Treatment: (2-4) 12x48x3” broadband panels on side and rear walls, floor-to-ceiling 24” wide super-chunk style bass traps.

Speakers: JBL 4722N Two-Way ScreenArray® Cinema Loudspeakers

Equalizer: Schiit Lokius

Subwoofer: MartinLogan Depth I

Amplifiers: Classe CA2100

Pre-Amp: Schiit Freya+

Pre-Amp Tubes: NOS Sylvania 6SN7GT on output stage, Tung Sol 6SN7GTB on input stage

DAC: Schiit Gungir Multibit

Phono Pre-Amplifier: Schiit Skoll (Phasemation T550 when running MC cartridges)

Turntable (Motor): Denon DP3000 housed in high mass plinth

Turntable (tonearm): Dynavector DV-505

Turntable (cartridge): Audio-Technica AT-VM740xML VMx Phono Cartridge
Click to expand...
not on subject, but since you're a fellow freya+ owner, i. thought id throw this out there.
them kenrad 6sn7s are no joke. they surpass my sylvania 6sn7 bad boy by a mile. absolutely sweet everywhere with a wonderful bottom end.
 
rothwellaudio said:
The OP is complaining about audible hiss without attenuators. How would placing attenuators before the preamp fix that?
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It will not fix hiss that is caused by the Classe amplifier itself. That was in response to the OP commenting on how they impacted the tone; the impedance mismatch was killing the sound. Based on the first post that stated the problem does not occur when using the Dynaco ST-70 the answer is already there - switch back to the Dynaco. While the Classe is newer and modern, I'm willing the bet that the Dynaco sounds a lot better with those speakers.
 
I have a similar problem in my rig with a high gain tube preamp driving a pair of Heathkit W-5M at 25 watts each, going to La Scalas. (Formerly had a Dynaco Stereo 70 with 35 watts per channel, but the Heathkits actually seem more sensitive.) I made a pair of L pad attenuators with a two RCA jacks, two RCA plugs, and four resistors. I made a simple Excel sheet to calculate the resistances I'd need to keep the 50k ohm impedance my preamp wants to see. I got lazy after making them and never made an appropriate housing so they are a little delicate hanging onto the inputs of my amps, but they work just fine. I may actually make a new set with more attenuation since I can't bring the volume control up past 25% without going deaf.
 
izeek11 said:
not on subject, but since you're a fellow freya+ owner, i. thought id throw this out there.
them kenrad 6sn7s are no joke. they surpass my sylvania 6sn7 bad boy by a mile. absolutely sweet everywhere with a wonderful bottom end.
Click to expand...
I’ve been lusting after the Ken Rads!! Where did you source them from?
 
techguy0192 said:
It will not fix hiss that is caused by the Classe amplifier itself. That was in response to the OP commenting on how they impacted the tone; the impedance mismatch was killing the sound. Based on the first post that stated the problem does not occur when using the Dynaco ST-70 the answer is already there - switch back to the Dynaco. While the Classe is newer and modern, I'm willing the bet that the Dynaco sounds a lot better with those speakers.
Click to expand...
The Classe is/was a loaner until my ST70 is back on its feet again but my friend asked for it back because he found a buyer so I just bought a Crown CE2000 to hold me over until I get the Dynaco back!
 
Ngen52 said:
I’ve been lusting after the Ken Rads!! Where did you source them from?i
Click to expand...
i got them from here.
iont know jack, but these sound soooo good.
i read many good reviews on them. mine are luscious.
im waiting for brentjessetubes to be back up. they have a nice selection, also.
the guy on ebay takes returns. had a great rating
 
As far as barrel type attenuators go, there are source connected and load connected attenuators. I have no idea what the difference is but mixing them up changes sound. The ones I have are Rothwell. Most attenuators are supposed to be source connected.
 
Fishee said:
As far as barrel type attenuators go, there are source connected and load connected attenuators. I have no idea what the difference is but mixing them up changes sound. The ones I have are Rothwell. Most attenuators are supposed to be source connected.
Click to expand...
A potential divider has an input and an output. An attenuator (which is a potential divider) could have its input wired to a female RCA connector (ie a socket) and the output wired to a male RCA connector (ie a plug). That's what we call an "inline attenuator", and it's what we sell most of. They're intended to be used on the input sockets of preamps or power amps or integrated amps.
We also sell what we call "source attenuators", and they have their input wired to a male RCA and the output wired to a female RCA. They're intended to be used on the output sockets of source components such as CD players or streamers or DACs etc. We introduced those primarily for the benefit of people with Naim amps or other equipment (usually vintage) which have their inputs on DIN sockets. We don't sell as many of those.
Yes, if you use the attenuators the wrong way round they won't work properly and will have a negative impact on the sound.
 
I use 6 10dB attenuators on the outputs from my Behringer DCX2496 in my power amps. These are balanced line devices which are designed for 600 ohm circuits. Even though they are feeding standard phono inputs, as was the DCX before they were fitted. The important thing to remember is compatibility. The DCX will run into almost any input impedance, balanced or single ended, quite happily, but that is because it is designed that way.

Any passive attenuator will reduce signal and any system noise present. Remember that any input treats signal and noise as signal and will amplify all of it. But all that aside, a passive attenuator should be designed such that it will attenuate without changing the impedance, but it must be selected for the type of signal it is going to be used with. A 600 ohm balanced circuit obviously has a different impedance than a conventional unbalanced circuit. So do you make it replicate the output impedance of the sensing device or the input impedance of the receiving device? Good question.

I think that with system that the OP is referring to, I would be inclined to make it replicate the high input impedance of the following amplifier as the preamp seems to be the more sensitive device.
 
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