Quickie with solid state amps

[markwilliams]

Hi everyone!

Can someone explain why my (stock) Quickie doesn't seem to work well with solid state amps?

Mine sounds great with all my Bottlehead and other tube amps, but with my old Hafler DH220 and Emotiva XPA-2 ....no good.    Sort of noisy/distorted.    Unlistenable.

Could this be an impedance mis-match?    And, if so, I have to admit I have NO understanding of impedance matching. 

Could someone explain this in layman's terms?  What is impedance?  Resistance?  And, how does one go about determining if a preamp will match well with an amp?

Thanks,
Mark

 

[Paul Joppa]

To talk usefully about impedance match in your system, we need to know the impedances. What is the input impedance of your "old Hafler DH220 and Emotiva XPA-2" amps?

Often when we talk about matching a preamp to an amp, we also need to know the signal voltage levels. So ... what is the sensitivity of the amps, and of the speakers?

Here's where I am going with these questions:

1) the speakers will likely be operated at 82dB level, with instantaneous peaks in the range of 96 to 102dB. Given the speaker sensitivity we can calculate the watts actually used. Give the amp sensitivity we can calculate the voltages needed to drive the amp.

2) Then we compare those voltages with the preamp's maximum output,

3) and with its noise floor.

If the amp needs more voltage than the preamp can deliver, you'll get distortion. If it needs a lot less than the preamp can deliver then it will amplify the preamp's noise floor and be noisy. In between is the sweet spot.

(The quickie can't generate hum on its own, but its tubes are often microphonic, so when I refer to noise floor, that's mostly microphonics.)

If the input impedance of the power amp is much less than 20K, then it will reduce the Quickie's peak output capability, so we'll do that calculation as part of step 2).

 

[markwilliams]

Hi Paul,

Thank you for responding to my questions. 

I found these figures on the two amps, but no more information than what I show.  Stuff in square brackets [ ] are my comments.

Here goes:

Hafler DH-220
115 watts per channel into 8 ohms
input impedance:  47K ohms
input sensitivity: 1.55 volts r.m.s. for 1 [then a space] 15 watts into 8 ohms
gain:  [no figure is given for this]

Emotiva XPA-2
300 watts per channel into 8 ohms
input impedance:  [unbalanced, which is what I use] 23.5K ohms
input sensitivity:  [I see no figure for this on the emotiva.com website]
gain:  32 dB


As I say...I have no real understanding of these figures.    I can tell you this.  I have a Conrad-Johnson PV10A tube preamp with 18dB of gain, works OK but a tad noisy background.    And a PrimaLuna Prologue 3 tube preamp with 12 dB of gain, and that works better.  Quieter background, etc.


Oh yes, the speakers:  Klipsch La Scalas with a sensitivity of 104 dB.

Thanks Paul,
Mark

 

[markwilliams]

Paul.........or anyone,

Thoughts on my post?

I provided information on those 2 amps.

Thanks,
Mark

 

[Paul Joppa]

Man, I'm sorry - I totally missed your post. Normally I use the "show unread posts" button to be sure I missed nothing - guess it didn't work this time.

Thanks for the info, it will be enough. The root of the noise problem is the use of powerful amps with sensitive speakers. For example, with 104dB sensitivity you can get 105dB from both channels using 0.63 watts per channel. That's the required loudness for a THX-qualified movie theater. Your amps can generate 200 to 500 times that much power!

The impedances are not a problem; Quickie can drive as low as 15K ohms with ease and small change in output. The problem is excess sensitivity downstream of the quickie. You turn the Quickie way down so the signal is small, but the internal noise and ringing from tube microphonics (tiny vibrations of internal parts) remains at the usual level. Those noises, along with the tiny audio signal, are enormously amplified into loud sounds.

The best solution is an attenuator at the power amp input, something like 20dB worth. See my article on signals and noise on the Community page of the website if you want to delve deeper or look for other solutions.

I did a quick google on the other tube preamps. The PrimaLuna Prologue 3 appears to use a step-down transformer output, which would act as an attenuator - probably why it is quieter. The Conrad-Johnson PV10A appears to be similar to the Foreplay in the line-amp circuit.

You mentioned distortion as well. I see no place for distortion to originate at such low powers,

 

[markwilliams]

Hi Paul,

Thanks for your answers.  I'd be very interested in implementing a gain control on the solid state amps.    Especially the Emotiva. 

I'm not sure how to do that.  In fact, I'm clueless.  I'd be open to a diagram or explanation.  I assume a potentiometer of some sort on each of the R and L channel RCA jacks of the amp.  But how to wire the pot?

Would a standard mono 100K volume pot do it?   

I've tried to do a Google search regarding adding a gain control to an amp and got nowhere.

I will try to find the information you mentioned.

Thanks again!

Mark

 

[Paul Joppa]

I'll do some numbers and list some options so you can think about it. Post back when you decide and we'll go from there.

The Emotiva is rated 300 watts into 8 ohms, that is about 50 volts output. Gain is 32dB, which is a voltage ratio of 40 (so it takes 1.2 volts to get 300 watts). You want to get about 1 watt from about 2 volts in, which is a gain of 3dB, so an attenuation of 29dB would be appropriate. The Haffler has 115 watts output which is 30 volts; at 1.55v input the ratio is a gain of about 20 which is 26dB. You still want 3dB, so an attenuation of 23dB would be about right.

There are three ways to introduce an attenuation:

1. As you suggest, use a potentiometer or level control. To get these high attenuations, an audio taper pot would be desirable; also I would use tow mono pots rather than a stereo pot, to allow for poor tracking at high attenuation. Or of course a switched resistive attenuator.

2. Use a fixed resistive divider (also called an L-pad) - if you know the desired attenuation, this is the easiest and cheapest - just two resistors per channel.

3. Use a step-down transformer. Attenuations of 12 to 18dB are readily available.

There are also three places to introduce attenuation in a system:

1. Inside the power amp, at the input terminals. The fixed resistive divider is the easiest. This placement is probably the best, but sometimes you don't want to modify a valuable amp, or don't want to make it permanent.

2. Add an adjustable level control, such as a

 

[markwilliams]

Hi there Paul!

I just read over your post quickly -- I will consider all the good ideas you gave later also as I read more carefully. 

YES, L-pad is what I was groping for / thinking of in the last post.    These are reasonably priced and probably easy to hook up and of course easy to disconnect.    100 watt?

Paul I do have a passive preamp I built with a simple Radio Shack Alps 100K stereo pot.    I set it up for 3 inputs and use a Radio Shack selector.    Maybe I will try this between the Emotiva and Quickie or the preamp du jour and see what happens.

I can tell you I tried that passive with the Emotiva when I first got the amp and then decided the tube preamps added just this nice touch of sweetness (of course!) so I went that route.

I would be open to opening up the Emotiva and Hafler and putting in fixed -- or better yet adjustable gain controls.    I'd need some guidance probably.    I know your time is limited so of course I'd be willing to pay for your guidance and instruction.  You could contact me off forum about that:  ms_williams@hotmail.com.    I expect a certain amount of help from this good forum, but not a riduculous amount.  Although I do understand some things from kit building, I consider my skills rudimentary.

Both of these solid state amps are very good.  My favorite amps remain my original Paramours and the Decware Zen.  But I like to run the solid state ones from time to time and especially on hot summer days so they are worth fooling with.    Perhaps an external box similar to the passive might be the quick answer, and if it works out I could then have things more permanent, or semi- permanent.

Paul, I don't know if you are Welsh or not.  If you are, happy St. David's Day. 

Thank you Paul,
Mark   

 

[markwilliams]

P.S.:

Paul, I did read your article on noise, etc. on the Community page.

Excellent stuff!

Thank you for it.

Mark

 

[markwilliams]

Hey Paul,

I got home for the weekend and hooked up my passive preamp between one of my tube preamps and that big Emotiva solid state amp.   

Result?  Success!    Very quiet now and I have much more control over the volume pot on the tube preamp.

I built some 3.3K ohm shorting plugs per Voltsecond's instructions and as Doc recommends here on the forum, and I placed them in the unused RCA inputs of the passive. 

Now, I'll build another little passive rig like this with of course no selector switch or extra inputs since they aren't needed.

If anyone else wants to try this it hooks up like this.  The interconnect from the tube preamp goes into the passive in the output jacks (these are the jacks where normally you'd hook up your amp if you were only using the passive preamp alone).  Then, the interconnect coming from your amp plugs into a set of inputs.  Naturally, make sure you have the volume pot on the passive turned way down before you.  Then, just experiment with the position of the passive's volume pot until you get the desired amount of swing on your regular tube preamp.   

I'm totally satisfied with this low cost solution. 

And I thank for your help!

Kind regards,
Mark

 

[Paul Joppa]

I'm glad it's working. I don't understand why the passive is hooked up backwards - I think it should work better the normal way around.

 

[markwilliams]

Oh Paul..........I'm mixing myself up!

You are correct.  Normal connections to the passive.

The tube preamp is considered an input, and then the amp hooks into the output.

I wish I could correct last night's post.

Mark

 

[Grainger49]

I have built about six of these with the same RS 100k Audio Taper pot.  Not to mention using them in a number of preamps.  I have a Creek remote passive between my FP 2, run almost wide open, and my Paramours just to get a remote volume in my system.  

If I knew how to make the RS pot motor driven and remote controlled cheaper, I would have done it.  But that wasn't possible.

The first attached photo is my Creek volume control.  This is not the fancy one with input selectors, just volume.  The Mermaid on top makes it sound better, really, it isn't the drugs!

And my second shot is my Mini-Mag Lite holder making fun of cable risers.  It moves the audio cable a little away from the Paramour power cord.  It didn't make any audible difference to me but if I cut the cables off I would have wasted two Ty-Wraps.

 

[markwilliams]

Hi Grainger!

Yeah, I was going to ask you about this but I decided to bug someone else for a change!

Russian teflons still sounding great in the Quickie.  Thanks again!

Mark

 

[Grainger49]

Good thing, attenuation requires me to take out the scientific calculator and review logs.  Logs still make my head spin and PJ is so much better with them than I ever would be.