The impedance question.

I have been told that if you have a tube preamp, and you have the option of changing the impedance at your amp, then a high impedance match is better than a low impedance. OTOH, if you have a ss preamp, then the low impedance is the way to go....Could anyone explain to me why this is the wisdom?

 

The impedance you speak of is the output impedance of the amp, which should be set to match the nominal impedance of your speakers. It's got nothing to do with the pre amp.

 

If OP means matching pre to power amps output impedance must be at least 1/10th ( ideally much less) input impedance of power amp. (Say power amp input impedance of around 20 times preamp output impedance). Many tube preamps have too high an output impedance to work optimally with solid state power amps. Generally tube gear works on higher impedance values than solid state but there are quite a few exceptions with preamp designs.

 

The original posting seems a bit confusing. Generally speaking, there is no way to "change" the output impedance of the preamp/linestage or the input impedance of the amplifier. Where there is an "impedance" option with tube gear has to do with the output of the amplifier where one can make a choice to better match with the impedance of the speaker being used with the amplifier. When matching speakers to an amp which has different output terminals, the choice is a matter of experimentation to find the best sound, not necessarily picking, for example, a "4 ohm" tap because the speaker is rated as a 4 ohm speaker; there is no "harm" in using the 4 ohm tap with an 8 ohm speaker or vice versa. The tap with the lower stated impedance will generally deliver more power but less ability to "control" the bass and maintain a flat frequency response.

As to the matching of tube preamp/linestage to an amplifier, as mentioned in other postings above, you generally want the input impedance of the amp to be at least 10 times the output impedance of the preamp/linestage. Because tube preamp/linestages typically have a higher output impedance than solid state gear, and because solid state amps typically have a lower input impedance than tube amps, there is more of a concern in matching a tube preamp/linestage to a solid state amp than any other kind of combination.

 

Sorry for the confusion. In my instance, I have a ss amp that has the option of either a low impedance or a high impedance ( done by switching) to match the up stream preamp. Since I use a tube preamp, I have been told to use the high impedance setting. My question is why does this matter? If the amp is set at too low an impedance, what is the issue with the preamp....and/or I guess the amp?

 

as i recall its to do with power transfer (may not be the correct term)
if your power amp input impedance is 10 times the preamp output impedance then you get a good power transfer with c90% of the power going to the power amp.
if you had the power amp input impedance = preamp output impedance then you'd only get 50% transfer
i think its something like that- happy to be corrected

 

Tube amps generally have a higher output impedance than their solid state counterparts. The higher the output impedance, the more difficult it is for the pre to drive the amp.

The higher the input impedance of the power amp, the easier it is to drive.

What is the output impedance of your pre?

 

There's voltage loss at the input. To minimize the voltage loss the input impedance should be at least 10X the source impedence. So if the output impedance of the preamp is 600 ohms, the input impedance of the amp should be at least 6000 ohms. There might also be issues with high frequency rolloff if the input impedance of the load is allowed to load down the source. Without knowing the specific numbers, but given that tube preamps tend to be higher output impedance devices than solid state preamps, I think the advice to use the higher input impedance setting on the amp is sound advice.

 

Impedance? Isn't there a little blue pill for that?

 

This is becoming more confusing because the OP does not provide any specs for the preamp and power amp impedance. In the great majority of cases solid state power amps have 20k or higher input impedance. So the tube preamp would have to have an output impedance well over 2k to cause any kind of issue. There are a few SS amps with 10k input impedance which might be more of an issue.

 

There are some far lower than that were you need a pre of 100 ohm output impedance or lower. Had this problem with matching Albarry Monoblocks and found a passive wasn't a good match either without introducing a buffer stage. Many people suggest valve preamps as a good match but very few are suitable. The mismatch in my experience is clearly audible. The 10X figure can be pushing a bit close and 20X is preferable.

 

DaveyF,

I took a look at our profile and it appears you are asking about the input impedance selector on a Rowland Model 8. You should use the high setting (36k) for a tube amp, and probably solid state linestages as well. The 600 ohm low selection might be better when a source component is directly fed into the amp (not using a linestage) or perhaps if the linestage has transformer outputs (rare). In any case there would be no harm in trying either setting to see which sounds the best.

 

Again most SS power amps are 20k or higher and almost all are 10k or higher. Lower input impedances than 10k are not as practical although designers can do any fool thing they want to . Passives are a special case which are hard to implement in most systems but a normal active preamp, tube or SS should easily be compatible with any SS amp having 20k input impedance and likely even with 10k impedance.

 

Thanks everyone. I am going to check the stats on my amp...Rowland 8 and my preamp...CAT SL1, and see what they are...will check back.

 

If it's the old SL1, according to this set of measurements, it's a 1k ohm output impedance: http://www.soundstage.com/measurements/preamplifiers/cat_sl1_ultimate_mkii/. The Renaissance seems to have a 100 ohm output impedance. Either way use the high input impedance setting on the amp.

 

The typical consumer interface and mentioned here is voltage transfer. That's the "10X rule".

In the tube and transformer era, it was matched loading to accomplish power transfer.

The difference is when it's loaded you have current flowing and have less sensitivity to metal junctions in the wiring interfaces. Rowland is one of the few wishing to continue the matching loading, power transfer method.

One of the things to consider is in the voltage transfer method and trying to do it passively...when you insert between the low output impedance and the high input impedance the voltage divider, the pot, then you have a much higher 'source' impedance driving that high impedance input. Plus, the 'source' changes at every adjustment of the pot. Hence, the passive's recommendations for short interconnect after it, etc.

To use the Rowland low impedance input, you need to be driving it with a source that is designed to drive it and indeed, thrives with that load. Then it can be very good. They could have labeled that input selector..."Us" and "Everyone Else". Of course that's not quite true, but not too far off either.

CJ

 

I am really not wanting to hijack this thread. Since the OP's initial post was a general question about impedance matching (preamp > amp) I hope I am not.

I thought I would start my own thread when I was ready to try this combination. (Not a good time for me to play around with this now.) Since this is going, I'll go ahead with my question now.

I bought an Audio Research SP6B from ebay several years ago. It looks mint, but I have no idea of the quality of the electronics. I think it is in original (old) condition. I got it for a good price and thought I'd send it to Audio Research for a restoration or freshening of components that wear out. I still plan to someday.

When I first bought it, I tried it out. All of the functions work. The sound was not right tonally. I thought it might be a function of an impedance mismatch. I had it plugged into a Harmon-Kardon PA5800 5-channel amplifer, using four channels to bi-amp Vandersteen 2Ci speakers. I used an Audioquest RCA splitter from the output of the SP6B and two long runs of Monster Cable M "something" RCA cables. It had very boosted bass and IIRC a somewhat rolled off high end. I had to turn the volume control up quite a bit more than I was used to. This was true of the line input and the phono input. I put it back in the box and then into storage.

Anyway, about impedance matching:

Audio Research SP6B Pre-amplifier
http://www.audioresearch.com/ContentsFiles/SP6B_Manual_Schem.pdf
Less than 500 ohms main output, 1000 tape output, Recommended minimum load for maximum audio quality 50k ohms and 250pf maximum capacitance.

Harmon Kardon PA 5800 Amplifier
http://www.manualslib.com/manual/65670/Harman-Kardon-Pa5800.html?page=10
Input Impedance 33K ohms

Doesn't seem like a good load, going by the (at least) 10x rule.

I have an Audio Fidelity X10-D Tube Buffer that I could use. (I did not try it in this application.)

Audio Fidelity X10-D
Input Impedance 470 K ohms
Output Impedance <200 ohms

Does this seem likely to be a good impedance match?
If not, should I forget this pairing of Preamp and Amp?
Or should I wait until I am in a position to try this out and start my own thread?

 

Well Audio Research can recommend anything but 33000/500 = 66 which seems higher than 10 to the naked eye. I believe the SP6 is a fairly low gain preamp. I don't know if that is an issue in your system. The SP6 is considered a very good vintage preamp so it may well be out of spec.

 

Thanks Mr Bass. I was picturing 500 ohms x 10 = 50k. Ordinarily my math is pretty good - not today.

So, it looks like I have an impedance match that seems okay. That is good news to me.

You are probably right in thinking my SP6B may be out of spec. I wouldn't be surprised if nothing has been done to it since it was built, back in 1980 or so. I was planning on sending it back to ARC before I actually put it into use anyway.

(The low gain is not an issue in my system. I just mentioned that since I was thinking it may be an symptom of an impedance match. This Tube preamp/SS amp and impedance matching is all new to me.)

 

But hold on...Dino. You mention using a splitter (a Y cable I assume) and long lines. So your preamp isn't seeing just a simple 50K load! And what it drives does depend on which end you have the Y cable too.

CJ

 

Thanks for the info. It looks like the older SL1 is at 102 ohms impedance at the output...not 1K ohm... did I miss something?

 

Your right I misread it. Either way 600 ohm amp input impedance seems too low.

 

The high impedance setting it is then.

 

I wondered about that. I used splitters like this:



I had it coming out of the back of the preamp and used two pairs of long interconnects.

Know of a way to figure the resistance that I am adding by these?

 

http://www.sengpielaudio.com/calculator-paralresist.htm