Amp, Preamp, two volume pots, there can be only one!


Hello everybody,

I have a question regarding the two volume knobs in my system and how they

should co-relate.

Here is my gear, both units are tubed:

Amp: Coincident Dynamo 34SE Mk1 (Output power: 8Wpc, Sensitivity: 300mV for full output, Input impedance: 100k ohms, S/N ratio: 92dB)

Preamp: Doge 8 2018 (Output impedance: 600 ohms, Input impedance: 50K ohms, Input sensitivity: 200mV)

Speaker: Proac 140 Mk1 (Sensitivity 91db, Amplifier Power Range 15 - 250 watts)

Now here is the dilemma, Coincident says if Dynamo is used a power amp the volume pot should be left open (heard that some call it a sensitivity pot too) but this introduces noise in the system (albeit when music is not playing) and leaves a very small usable range on the preamp volume knob, and hard to hit the sweet spot as at 9 o clock its already very loud.

So what would be the best way to run things without losing signal quality? And is there any mismatch between my power/pre?

Thanks in advance for any suggestions!

neverlast

My amp has a 100k volume pot and I took my preamp out.

It had an Alps, but I changed it out to a TKD.

Since I have two sources I got a custom switch box.

Less stuff in the signal chain the bettter I say.

When I did run a preamp, I believe I did the same as you.  Ran the amp wide open and adjusted volume via the pre but I started thinking that it was redundant.

Just gotta try and see what works for you.

It’s not a serious mismatch per say, it just appears you’ve got a high gain tube preamp without the highest signal-to-noise ratio. Your Doge lists "200mV sensitivity" - if that is in reference to a 2V output, that’s 20dB gain which is very high. Generally, when you get a tube pre up in that range you’re gonna hear low level hiss noise at output (from speakers) unless your speakers and/or amps are low sensitivity. Your amp’s gain (27.6dB) and speakers (91dB) are sufficiently high that this noise floor is totally expected. Tube power amps almost always have pretty high sensitivity / gain - even low power ones like yours! 

So now it’s just whether the noise bothers you enough to cut it down with that amp potentiometer. You might try a little and see if that sounds "transparent enough" for you.

Thanks for the input guys,

@mulveling the noise does not bother me much when the music sounds good so thats not really a problem, only problem is that Im controlling my volume with the preamps remote and the jumps are too high when the amp is fully open. Read somewhere that the pots balance is usually better in the later stages.

@theflattire I would also consider using only the amp but I have 4 sources connected one of which is the phono and since the preamp has the section I would need to alter my system quite a bit.

 

Try backing off on the amp volume control to where you can use the preamps volume between 9 and 12 o'clock.  That may eliminate the noise while not influencing the sonics. Another approach might be to have the preamp modified to reduce the gain if that is possible. 

There's no technically best answer here, since the actual gain isn't being adjusted.  I think @mesch answer is pretty sweet though.

That is, I don't think the amp volume control will affect the character of the sound at all, just noise, so make yourself comfortable and find the range that works for you.

It's generally true though that most preamps have much more gain than we ever really need, some of that is historical, so you can turn up the volume to hear the faintest of signals on a radio, but in modern times I think it's time for that to be rethought.

@neverlast With full output at only 300mV your amplifier is far too sensitive. Since it can only make 8 Watts, it has to be used on a high efficiency speaker. So you'll be fighting noise. So yes, that's a bit of a mismatch!

Usually SETs (or SEP in this case) have low gain so as to deal with this issue.

I rarely recommend a passive setup but this appears to be a situation where it would be handy. OTOH, finding an amplifier of similar spec that has a more real-world input sensitivity would help with the noise and you may notice greater low level detail as a result.

A volume control can't alter, distort, overdrive, add noise, etc.. to the signal, other than the impact on sound quality the control itself has. (Gain controls can, but not volume.) It can't add or take away noise. The problem lies elsewhere in the system. That said, all audio equipment is prone to the type of noise you are talking about under certain conditions. It doesn't mean anything is wrong. The simplest and easiest solution is to get some Rothwell attenuators that just go inline on a pair of interconnects. They are made for this specific problem and you shouldn't hear any difference in sound at all. 

r1g450

A volume control can't alter, distort, overdrive, add noise, etc.. to the signal ...

I couldn't disagree more. It isn't clear why you'd make such a sweeping claim.

Like others have said, run up the amp to the noise floor.

Let the pre do the rest.

Thank you all kindly for the ideas and advice, I will give everything a try.

@atmasphere in theory which specs would the amp need to have to play along with my preamp?

 in theory which specs would the amp need to have to play along with my preamp?

@neverlast Find one that takes 2Volts or more for full output. If the amp is properly designed, that will drop your noise floor about 15 dB and will make your volume control a lot more useful.

Thanks @atmasphere will keep this in mind in case I can try something in my system and compare. My Doge preamp has a switch which can drop the gain by -8db, would this be useful in my system and would it have some derogatory effect on the sound?

My bad, it appears that the -8db gain reduction works only for the phono stage…

Short update: After listening to several positions it seems that there is something lost in the high frequencies when I set the power amp at 3/4 which leaves a useable range on the preamp. Would using an attenuator (cca -11dB) be a better option or would it be the same thing in essence?

I solved that problem with a Schiit Kara preamp, but any good quality passive preamp will work. I simply run the output of my Microzotl pre into an input. In passive mode the volume control acts as an inline attenuator. I set mine about 2 o'clock which gives me enough room on my preamp volume control. It sounds better than the attenuator on my power amp. And it's easy to open the Kara full and essentially remove it from the chain. 

An added bonus is conveniently switching to the SS mode of the Kara for casual listening.