[sonofnotredame]

Hello all,

I’m trying to figure out the input impedance for my power amp in order to determine if a passive preamp will be a good fit.

Power amp: Uher UMA-2000 (manual available on hifiengine)
Preamp: Mod Squad Line Drive

Input impedance requirement: >= 47 kOhms

Thanks for all your help.

[FullRangeMan]

If your amp impedance is 47K currently rule of thumb used favor a preamp about 1 to 5K ohms range.

[sonofnotredame]

Thanks for the response FullRangeMan. I’m not sure what the input impedance is, that’s the problem. The preamp is passive. The preamp’s designer recommends that the power amp have an input impedance of around 47 kOhm. Before I buy the preamp, i want to make sure my power amp will be a good match. Sorry for the confusion.

[Rusty Jefferson]

It looks like 50k ohms input.

http://www.hifi-wiki.de/index.php/Uher_UMA-2000

[FullRangeMan]

Its preferable a preamp on the lower side 1-2K side, I could look a tube preamp on Ebay, a 6922/64J8 tube has many substitutes to roll.

[Rusty Jefferson]

I assume you've read the Stereophile review. It details the importance of matching your source(s) and cables with the preamp.

[sonofnotredame]

I have read the review, but perhaps I’d better read it again now that I’m serious about buying the preamp. Thanks for the info on the input impedance. It sounds like the pre/power amp will be a good combo spec wise.  My source will be an iPhone going through an MHDT Havana DAC.

 I’ve got some nice, handmade interconnects that were a gift when I bought the power amp, but I’m not sure what their specs are. They are about 4 feet long. If I need new cables, I’d be willing to buy them once I hear how everything sounds.

I really do appreciate all the input and help. FullRangeMan, I have a feeling that a tube pre will be out of my price range and the power supply compatibility between Germany and USA will limit my choices even further.

[FullRangeMan]

You can order 230V and after rewind the power transformer primary to 120V or even a new improved power transformer, tube amps with point to point wiring are all modular.

[Speedskater]

In analog audio, the cables have nothing to do with impedance matching.
That's a digital audio thing.

[richidoo]

My source will be an iPhone going through an MHDT Havana DAC.

Your DAC has a tube output stage. You should find out what output impedance the DAC has. It will be the only source of current to power the passive preamp, the power amp input stage and the 2 runs of cables. The passive preamp probably has 10kOhm input impedance so you'll need good current from the DAC to drive that and avoid loss of signal. Cables do have an effect on signal if the source current is inadequate to drive them. You could measure the resistance and capacitance of the cables to make sure they are similarly low compared to quality commercial interconnects. Both metrics are proportional to cable length, so keep them short!

AC member Pez made a nice post last year about the tube in your DAC. It is an earlier variant of 6922, which is a very powerful, high current tube. So you should be OK as long as the tube circuit in the DAC makes the most of the tube's current ability. But that's just my guess, based on Pez' info, so I recommend that you find out for sure what is the DAC's output impedance from the mfg, and ask what's the highest current tube the DAC can handle. I would look for output impedance of the DAC to be <600 Ohms for best results in your system. Technically, the lower the impedance the better, but if you like a "tubey" warm soft sound then a higher impedance (DAC Zo=1k+) ratio will give you that.

When you get it setup, try any old CDP with solid state output stage, or even your iPhone direct to the preamp without the DAC. If it sounds noticeably stronger bass and more detail then you might have an impedance problem with the DAC.
Rich

[Rusty Jefferson]

Agree with Rich's comments. It looks like the dac is rated at <800 ohms of output impedance, if this is your model?  Keep your cables short.
http://www.enjoythemusic.com/magazine/equipment/1208/mhdt_labs_havana.htm

How does one connect an iPhone to this dac?

[sonofnotredame]

Thanks Rich and Rusty for your excellent posts and great information. I wrote to MHDT labs to see if I could get an exact figure of output impedance. Isn’t it possible to adjust the output of the source (iPod) by raising the device’s volume? It looks like I’ll have to play around with it.

Connection options would be USB from the iPod to the DAC. I could also go TOSLINK from an old appleTV to the DAC if necessary.

This is a copy of what the designer of the preamp talked about when matching an amp to the preamp. Unfortunately, nothing was discussed regarding sources. I’m not sure if someone smarter than me could glean more from it though.

“I hope you enjoy your Line Drive and find a good amplifier match for it. When looking for appropriate amps, keep the following general recommendations in mind:

1) Look for a fairly high input impedance - around 47Kohm or higher. Anything below 20Kohm is not the best match, although it will work.

2) Fairly high gain is also useful (in the range of +26 - 30dB, for instance). You may see this expressed as "input sensitivity," which is measured in Volts. Look for a figure around 1 Volt, give-or-take a bit. Anything in the 2 Volt-or-higher range may not give you the system volume you would like.

3) Minimize the capacitance of the interconnect cables between the Line Drive and the amp. I suggest 1000pF (pico Farads) as a *maximum* in this connection, with less being better. You should be fine with most cables of 2M or shorter.

Check with the FAQ section of my website at www.SMcAudio.com for further information.

Have fun!

Best regards,

Steve McCormack
formerly with The Mod Squad”

[Rusty Jefferson]

I had no idea one could send music over usb from a phone to a dac.  Learn something new every day.   

[sonofnotredame]

I also have a headphone to RCA cable if the USB doesn’t work. I’ve had mixed results with audio via USB so that’s the second option.

[richidoo]

Isn’t it possible to adjust the output of the source (iPod) by raising the device’s volume?

Volume is voltage, it is different than current. The source (output) impedance describes the current available to drive the signal. If the impedance ratio between source and load impedances is 1:10 or larger, then you know you have enough current to preserve the integrity of the signal through that leg of the signal chain. All legs should have 1:10 or greater impedance ratio to preserve signal integrity through the system.

Check out the measurements page of that Stereophile Line Drive review, it goes into detail about impedance issues of the Line Drive. You are the guy they're talking to! 

With 800 ohm source impedance from the DAC driving 8200ohm preamp, that's 1:10 so you should be OK there.

The Line Drive, being a passive volume control has varying output impedance. The minimum is 30, max is 2050 ohms. Add the max to the 800 from the DAC, plus worst case 100 ohms for the cables and you have 3k Ohms driving 50kOhms amp. That's still better than 1:10 so you're fine there too.

If bass feels lacking you can improve it by increasing the size of your DAC's coupling caps. This calculator suggests you would want a 10uF cap to drive the Line Drive. Tube output stages usually come with 1uF - 2uF caps, because they assume you will be using an active preamp with at least 50k-100k input impedance, not 8.2kohm impedance like Line Drive. Bigger caps would better adapt the DAC to the Line Drive. A pair of nice 10uF caps that sound good enough for that DAC prolly around $50-100?  Hopefully you won't need that!
Rich

[sonofnotredame]

Thanks Rich for the explanation. Just a couple follow up questions...wouldn’t adjusting the output voltage of the source by raising the volume have an effect on the input current? Nothing has changed to the load, right? I look forward to getting the components so I can hear how everything sounds.

[richidoo]

Yes, ohm's law describes how when you add more voltage to a constant load then more current flows. Your preamp has constant input impedance so turning up the DACs output voltage via the iPhone volume control would increase the current demand on the DAC tube. But the DAC can't see what happens after the preamp. The DAC tube is only concerned with powering the load it sees, which is constant 8200 Ohms, no matter what position the preamp volume control is set.

Changing the preamp volume control will change the voltage sent to the amplifier, but it also affects the source impedance of the preamp because of how volume pots work. Active preamps use an output buffer for this reason, to ensure the output impedance is low and steady no matter what position the upstream volume control is in. With varying source impedance into constant load impedance you have variable impedance ratio. If the ratio gets too low then you can start to have signal degradation. But even in your worst case the preamp's output impedance is <3kOhm so you will always maintain favorable impedance ratio to the amp, you should not hear any difference in SQ between 2 oclock volume knob and full open volume.

But you can imagine an example where the amp's load impedance was only 10k and a solid state DAC has 100ohm source impedance. At wide open volume the pot's output impedance is 130 ohms which can drive the 10k amp easily. Turning down the volume makes the pot's output impedance rise up to 2150ohms, which would probably audibly degrade the signal going to the amp.  The impedance ratio would change from 1:50 to 1:5.

[sonofnotredame]

I just received the email response from MHDT Labs. They said the output impedance of ge DAC is about 32 ohms. That seems an order of magnitude less than the numbers we were discussing. Am I missing something? In any case, I’m at the German post office waiting to pick the DAC up so I’ll see how it sounds in the system later tonight.