A Few Questions

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Housteau

A Few Questions
« on: 9 Mar 2009, 08:39 pm »
This is my first time posting here on this forum, although I have been a Circle member for a while.  I have to admit that I don't know a lot about passive preamps.  However, many years ago I did own the original Line Drive from Mod Squad.  From my reading so far here and from other mentions elsewhere, I see that that these newer designs are an entirely different animal all together.

My system is currently passively triamped.  Well, that is to say my main towers (VMPS RM-V60s) are passively biamped, and the separate bass towers use mono amps with built in crossovers.  Still, my tube preamp is seeing the load of three amplifiers in parallel.  The total impedance is very low, a lot lower than tube preamps want to see.  I have helped out this situation by taking the sub bass amps out of that parallel circuit by running them directly from the output of my higher frequency VTL monos.  To do this I made a bridging circuit, or a voltage divider IC dropping the VTLs output back to line level.  Now my preamp only sees two amplifiers in parallel for each channel.

This arrangement may change in the not too distant future by going active instead of passive, but for now this is my world, and I like my world for the most part.  My preamp was an Audio Research, but I am now using the built-in preamp of my Monarchy NM24 tube DAC.  I think this is an excellent DAC and the preamp isn't bad either.  But, I have read that its volume control takes away from an otherwise excellent line stage, and by using something like what you offer here will really increase it to its full potential, and the differences are not subtle.

So, that brings me to a few questions.  What about this impedance issue of driving several amps in Parallel with an 18' interconnect?  What is the lowest amplifier input impedance your controls can see without damage to the music?  Would step up transformers be helpful in bringing up that total impedance values?

John Chapman

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Re: A Few Questions
« Reply #1 on: 9 Mar 2009, 09:40 pm »
Hello!

Thanks for the post!  I'll do my best to reply as clear as I can- I seem to have a way of making things more confusing than they really are.....

Your question revolves around how well you'll drive downstream loads - those the the pre-amp 'sees'. The Autoformers do a better and better job of driving loads as you turn the volume level down from the top settings. Once you get down even a bit from the top this affect really kicks in and after a certain point it becomes academic - since once it is doing a really good job after that any better job of driving the loads is not likely a sonic factor anymore.  It's this lowering of output impedances that is largely responsible for the sonic differences folks hear when comparing to older resistor based passives. This does not mean resistor passives don't work well - they can work well - but it does help when the loads get tougher.

Because the answer relates to where you would have the level control set the source's voltage and output impedance and also the speakers and the amp sensitivities all come together to set up the environment the pre-amp will run in. Note that in most cases all this is just fine - it's only in the fringe of situations where things can be an issue.



I just had an e-mail from a fellow with the same DAC yesterday - very nice looking unit. There was a review (likely the one you saw) by Lynn Olson that stated he had better results with Dave Slagles autoformers in-line rather than the internal pot.  If Lynn thinks it sounds better I would not argue - he's a great guy and very particular of sonics! I am not entirely clear on how exactly that was hooked in - the dac is very flexible in the way it allows hook-up options! If I read it right he inserted the autoformers in a loop of sorts - with the dac output feeding the autoformers and then with the output of the autoformers looping back in to the Dacs internal line stage. I'm pretty sure that is the way it was.

If that is the case the autoformers are actually driving the line stage inside the dac  - and therefore the autoformers  never see the downstream loads like the cables or amps. The line stage becomes a buffer that blocks the connection from the autoformers to the amps and takes over the job of driving all that stuff.  In this case the system should do as good a job driving everything as it does now - the level control upgrade is what would really be added into the mix.

If the autoformers were inserted into the system after the dac as a stand along pre-amp (another way to hook them up) then we'd need to see the specs of the dac output to really confirm where things would be at as far as driving very low loads and quite long cables.  I could not spot the output specs for the dac - if anyone can point me to them that'd be great! This information would be used to estimate how far down from the top level settings you'd be running the pre-amp and as a result  how well you'd be able to drive the loads.

Now that I once again failed to keep things simple I'll end with specific answers to your questions:

>>>>>What about this impedance issue of driving several amps in Parallel with an 18' interconnect? 

A few amps in parallel are seldom an issue - as long as you are down a bit from the top levels I am actually surprised at how well the autoformers drive low loads.  Unless you get really extreme (like a net load under 5K) it's no issue at all.  Even in cases like that it can be fine if you run down from the top volume levels and if the source has a decent output stage. I tested driving 600 ohm loads some time back and was surprised how well that worked!  You could clearly see the measurements get better and snap right into where they should be as the level control was turned down.

I am a fan of shorter cables so when they question is about long cables I can only fall back on what users have told me and what the measurements here would predict. Just like the loads the cable load gets less and less a factor as you turn down the volume - that impedance lowering affect of the autoformers is helping out.  I have had customers run long cables with good results - similar lengths at times and often these are guys who use active speakers like ATC's, etc...



>>>>>>>>>What is the lowest amplifier input impedance your controls can see without damage to the music?

As mentioned above - pretty low is o.k. I;d say when we get under 10K we should then look at the source and speakers and amps etc to confirm all will be well. Above that it's not an issue.


 >>>>>>>>Would step up transformers be helpful in bringing up that total impedance values?

Actually a step-up would be the last thing you'd want. Autoformer volume controls do a great job as you turn the volume down but if you step up the levels the impedance lowering affect works the other way around - against us in this case! A step-up would make it less tolerant of the downstream loads.

As mentioned up above in your particular case - if you are looking to hook-up like Lynn did in his review of the dac -  the autoformer would be just fine in there. If the system is driving the loads fine now from the line stage then all would continue to be fine after the change.


Thansk!

John





Housteau

Re: A Few Questions
« Reply #2 on: 9 Mar 2009, 09:59 pm »
Thank you for your quick response.  You nailed it.  That review is exactly the one I was thinking about.  In it he states:

"I quite seriously recommend using the M24 with an external top-quality control—and chassis mods are not necessary, since the necessary inputs and outputs are already there on the rear chassis.

If you use the Linestage with the built-in control, it'll sound noticeably more closed-in, less dynamic, and less open than it really is, with just a bit of grain at the top. With an external AVC or TVC feeding the linestage, the linestage actually sounds better than the AVC or TVC by itself, and the sound quality leaps to the top of the tube-linestage league. The real giveaway is the grain—a good tubestage should never have any grain, at least with a competent power supply. Get rid of the control, the grain completely disappears, the soundstage width and depth doubles, the bass deepens, and dynamics really open up. It's not a small difference."

You are correct in that the DAC signal can be looped out and back into the line stage.  But, by doing that the signal still passes through the original volume control and I would think that would not be ideal.  So, I must still be missing something here. 

John Chapman

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Re: A Few Questions
« Reply #3 on: 9 Mar 2009, 10:19 pm »
Hello!

I wondered the same thing about the pot and the signal path - but Lynn runs circles around me when it comes to analog designs so I figured I was just missing something! The dac has quite a few hook-up options and I have not really worked through them all...... If indeed it does still have the pot in the path turning it up would allow a test and then if it were me I'd replace it with a fixed resistor  for long term use that way.

Thansk!

John



1000a

Re: A Few Questions
« Reply #4 on: 3 Apr 2009, 10:45 pm »
according to the NM24 designer the output impedance of the unit is 300ohm.

hope this helps- trying to get my NM24 into my audio chain also

1000a

John Chapman

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Re: A Few Questions
« Reply #5 on: 4 Apr 2009, 02:20 pm »
Hello!

The 300 ohm spec confirms that the unit would do just fine driving the autoformers - or any other TVC or autoformer I would expect. Lynn's review would confirm that - since the autoformers worked very well with the NM24 based on the comments.

The trick is to figure out just how they strapped it into the signal path. Based on what I know they likely just turned the pot right up and then used the autoformer  looped in the signal path between the DAC output and the buffer input.  Best would be to get the pot right out I expect but I don't see any way to do that without yanking it out of the unit - and they specifically state in the review that they did not have to mod the unit.....

I did e-mail Lynn but have yet to hear back from him. Not sure if he is around or online these days, etc....

For you guys with these NM24 units - just try the outputs and inputs to get a handle on what works best!

Thanks!

John

1000a

Re: A Few Questions
« Reply #6 on: 6 Apr 2009, 08:30 pm »
John,

I wanted to take the time to thank you for your kind help and quick reply. 
Also thanks a mil for sending Lynn Olson an email I had thought of doing
the same because like you I could not get a handle on exactly what he did
with the existing vol knob 3/4 up and so forth. 

Anyway many thanks let us know if you hear from Lynn.

 :D

Clio09

Re: A Few Questions
« Reply #7 on: 21 Apr 2009, 04:21 am »
I also had the same DAC at one time and was using a K&K TVC with S&B TX MkI transformers. I discussed the loop concept with Thom Mackris at Galibier Design who is friends with Lynn Olson and in whose system they tested out this DAC/pre with the autoformers. According to Thom the idea was to set up the autoformers in the loop, DAC output to autoformer input, then autoformer output to DAC/pre input. Outputs from DAC/pre to amp. Once done the next step was to pin the volume control on the DAC/pre to wide open, then use the autoformer to attenuate.


John Chapman

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Re: A Few Questions
« Reply #8 on: 21 Apr 2009, 08:59 pm »
Hello!

Great info - Thanks for posting that. It makes perfect sense that way and the only step beyond that would to get diy and bypass the pot inside, etc.

Thanks!

John