[TreAdidas]

Howdy All,

Got a question regarding the output sensitivity of the MPX3 when used as a preamp.

I purchased an MPX3 with the notion that I would use it as a preamp 80% of the time and a headphone amp around 20% of the time. I wanted to bypass the preamp portion of my vintage Sansui AU-20000 integrated amplifier. This integrated amplifier is an overall formidable performer, but the preamp section is downright noisy. Luckily the preamp and amplifier sections are fully discrete and connected via a jumper which is easily bypassed. As such, there are RCA inputs on the back of the amp that feed directly to the amplifier section of the unit.

Unfortunately, when I hook the MPX3 up to my amplifier (with the preamp bypassed of course), the volume is outrageously loud even with the MPX3 set at the lowest volume setting. Additionally, the microphonics are unbearable. Merely running my finger over the volume pot yields tangible to borderline potent microphonics even with the MPX3's volume at zero.

I attempted to correct the problem by inserting a 12db inline attenuator (link: Parts Express:Harrison Labs 12 dB RCA Line Level Attenuator Pair) between the MPX3 preamp outputs and the RCA "main" inputs on the amplifier section of the Sansui. This offered negligible improvement in both volume and microphonics.

I do not know the input sensitivity nor impedance of my amplifier and do I know the output sensitivity nor output impedance of the MPX3. I am by no means an electrical expert but are these possibly my culprits? Moreover, is there anything I can do to clear this up or am I just out of luck as I am trying to pair up two unpairable pieces of equipment?

 

[Jahvetti]

I have experienced similar problems when using Supra as preamp. The output level seems quite high for solid state power amps. If someone has a working solution to this, I would be thankful.

 

[sacd lover]

Quote:

Originally Posted by TreAdidas /img/forum/go_quote.gif Howdy All, Got a question regarding the output sensitivity of the MPX3 when used as a preamp. I purchased an MPX3 with the notion that I would use it as a preamp 80% of the time and a headphone amp around 20% of the time. I wanted to bypass the preamp portion of my vintage Sansui AU-20000 integrated amplifier. This integrated amplifier is an overall formidable performer, but the preamp section is downright noisy. Luckily the preamp and amplifier sections are fully discrete and connected via a jumper which is easily bypassed. As such, there are RCA inputs on the back of the amp that feed directly to the amplifier section of the unit. Unfortunately, when I hook the MPX3 up to my amplifier (with the preamp bypassed of course), the volume is outrageously loud even with the MPX3 set at the lowest volume setting. Additionally, the microphonics are unbearable. Merely running my finger over the volume pot yields tangible to borderline potent microphonics even with the MPX3's volume at zero. I attempted to correct the problem by inserting a 12db inline attenuator (link: Parts Express:Harrison Labs 12 dB RCA Line Level Attenuator Pair) between the MPX3 preamp outputs and the RCA "main" inputs on the amplifier section of the Sansui. This offered negligible improvement in both volume and microphonics. I do not know the input sensitivity nor impedance of my amplifier and do I know the output sensitivity nor output impedance of the MPX3. I am by no means an electrical expert but are these possibly my culprits? Moreover, is there anything I can do to clear this up or am I just out of luck as I am trying to pair up two unpairable pieces of equipment?

I know that is definitely the problem. I have experienced this problem myself. A lot of the newer SS amps can be driven to full output with < 0.5 volt. In my case, I used the Monarchy SM70 Pro which has an input sensitivity of 0.4 volts for full output. The SP preamp output voltage just overwhelms an amp in this case. I too tried inline attenuators with no luck.

The only solution that ever worked for me was to use a passive preamp after the SP. That way I could knock down the output down, get some useable volume control range and still retain some of the tube sound. I specifically used an Axiom passive preamp from Luminous Technologies. I still have one .... and they are inexpensive at around $100 used. This is certainly not an optimal solution. But, I did get acceptable results.

 

[TreAdidas]

Thanks to the both of you for verifying that I am not crazy!

Quote:

Originally Posted by sacd lover /img/forum/go_quote.gif The only solution that ever worked for me was to use a passive preamp after the SP. That way I could knock down the output down, get some useable volume control range and still retain some of the tube sound. ....This is certainly not an optimal solution. But, I did get acceptable results.

You stated my exact goal: "knock the output down, get some useable volume range and still retain some of that tube sound."

My workaround thus far has been to engage the preamplifier section and use "dualing volume knobs" similar to what you described except for that my preamp is an active noisy one. Bleh.

So this begs the question - if one has a stepped attenuator, which I fortunately do, can the values on the attenuator be adjusted to do this in the MPX3 (or any other singlepower unit for that matter) itself? Seems to me we would essentially be doing the same thing as sticking a passive volume control in between the preamp and the amp but without the additional cabling. Bonus points!

Even when I find myself a few beers deep and listening at what I would otherwise peg as irresponsible levels, my volume pot is never past the 10:00 position for headphone use. Under normal conditions it is usually around 8:00. Now, the volume pot itself starts at around 7:00 and clicks all the way around to approximately the 4:00 position. I verified this with the unit powered down of course. As far as I am concerned everything from 10:01 - 4:00 is useless for both headphone and pre-amplification applications as it is just too loud. Heck for all I know anything past 11:00 may be blank. I wonder if Mikhail could insert some higher value resistors (may be using the incorrect term there) into the lower positions on the pot (the 7:00 - 12:00) thereby making the output power come up at a more gradual level. Then what is not my first step could actually start at 12:00. I have no problem putting the pot to 12:00 for headphone listening if I gain the ability to use everything before that for my when I am using the unit as a preamp.

Anyone see any issues with this? Is it possible that the input would then be too low for the tubes to function properly? I'm not sure this remedies the microphonics issue but I'm willing to give it a shot either way. I'll give Mikhail a call on my much break today and see what he thinks.

Standby!


Oh and FYI: volume knob = pot = attenuator. Those terms are all synonymous. This confused me when I was getting started out so I thought I would clarify.

 

[darkless]

On my future Singlepower amp I have had a "gain" knob installed which allows me to dial in additional attenuation from -40 to 0 dB in 10 dB increments. This should hopefully be enough to ensure a proper use of the volume dial.

 

[Fixcinater]

From my limited experience with audio circuit design (mostly limited to guitar tube amplifier circuitry), it sounds like the pot needs to be swapped for either a different value (which would affect the total gain possible, obviously not a problem being that you aren't complaining of it being way too loud, switching sources and then needing more gain) or a different taper of the same value.

Check out logarithmic versus linear tapers here for more info: Clicky! (about half-way down the page).

This is probably a function of the amplifier being designed firstly for that function of full headphone amp, with only secondary (if any) regard for it's preamp-only usage.

 

[Head Whipped]

Do any of you experience microphonics when listening in headphone mode with you SP amps?

 

[nikongod]

Quote:

Originally Posted by Fixcinater /img/forum/go_quote.gif From my limited experience with audio circuit design (mostly limited to guitar tube amplifier circuitry), it sounds like the pot needs to be swapped for either a different value (which would affect the total gain possible, obviously not a problem being that you aren't complaining of it being way too loud, switching sources and then needing more gain) or a different taper of the same value.

the SP amps all use a log pot.

the issue here is toooo much gain. whether it is in the SP amp, or the power amp remains to be seen

i suspect the power amp has more gain than it needs, so that the preceding stages in the system can be made less expensively.

if you are DIY handy, you may want to try to build the attenuators using lower total values. a 100 ohm could be worth a shot. it could either work better or worse depending how mikhail sets up the preamp output.
Quote:

Originally Posted by Head Whipped /img/forum/go_quote.gif Do any of you experience microphonics when listening in headphone mode with you SP amps?

only with certain tubes.

with some tubes you can almost hit the amp with a hammer and get no microphonics. other tubes are microphonic when the wind blows hard.

 

[Head Whipped]

Did you get them with the stock tubes?

 

[nikongod]

Quote:

Originally Posted by Head Whipped /img/forum/go_quote.gif Did you get them with the stock tubes?

the electro-harmonix tubes i have gotten my hands on have generally been good.

if you are running 3X 6SN7, try swapping them around. even if they are differently branded. its a quick test. the input/gain tube is the most important for microphonics.

 

[TreAdidas]

SO I chatted with Mikhail today and there is a fix he can do with swapping out the volume pot and putting on in there wiht a higher slope. He gave me two options one was where a single resistor is wired at each point and the other is where the resistors are in a series. The cost was $550 and the other $350.

My first reaction was an exuberant let's do it! Then I started thinking about it more. I traded my PPX3 + $700 for the sake of adding preamp capability and improving my headphone amp. So I get preamp capability that does not really work properly and I need to spend another $350 to get it to work?

Ehhh it is making me kind of blah about the whole experience.

Perhaps Fixinater is right. I am asking something that was designed primarily as a headphone amp to be primarily a preamp. While I am in lust with Singlepower's stuff, perhaps I should rethink my approach.

 

[TreAdidas]

*Duplicate post.

 

[TreAdidas]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif the SP amps all use a log pot. the issue here is toooo much gain. whether it is in the SP amp, or the power amp remains to be seen i suspect the power amp has more gain than it needs, so that the preceding stages in the system can be made less expensively. if you are DIY handy, you may want to try to build the attenuators using lower total values. a 100 ohm could be worth a shot. it could either work better or worse depending how mikhail sets up the preamp output.

I hear ya on the too much gain. The input on man Sansui is really sensitive. It makes sense as it was manufactured circa 1975 and the sources at the time were no where near as powerful as they are today.

Unfortunately I am not very DIY handy just yet. Thinking of trying my hand at terminating my own speaker and interconnect cables, but other than that not so much.

 

[Fixcinater]

Chuckie-

I'll trade you the 325is for the mod work.

Seriously though, could you find a replacement for the Sansui since it is on it's death-bed anyway (who wants to spend $350 to re-cap a 35 yr old integrated that wasn't super hot/classic to begin with?), that would be more flexible on input volume from the preamp?

That is, keep the MPX3 as-is and work around it. I could bring the Creek down and we could see what happens with a more standard (modern) example of amplifier.


OR...That same money could go into a power conditioner for you.

OR...get the darn DAC 1 and sell me your MPX3.

 

[TreAdidas]

Quote:

Originally Posted by Fixcinater /img/forum/go_quote.gif OR...get the darn DAC 1 and sell me your MPX3.

Alas you bring up an interesting point. The DAC-1 does have those pads on the outputs and thus I could bandage this gain issue by reducing the outputs on the source I suppose.

Bleh - I have to let this stew for a little while. I *want* the whole hooked up MPX3. Rationally, however, my What-O-Meter is going off about spending $350 to fine tune the preamp on an already $1100 piece of equipment billed as a preamp. The ID and the EGO are having it out on this one.