Has anyone bi-amped with a Cary SLP-05?


I have this rather weird problem I am hoping the collective wisdom of Audiogon will be able to help me sort out.

The SLP-05 preamp has a pair of RCA outputs, and a pair of XLR outputs. I recently tried bi-amping using both outputs, and the result is the SS power amp always fails to work, no matter what output it is connected to. Here are the configurations which I tried:

SLP05-> (RCA)-> Cary CAD2000-> speaker
(Full range). Result: success

SLP05-> (XLR)-> Cary CAD200-> speaker
(Full range). Result: success

SLP05-> (XLR)-> Cary CAD211AE-> mid/tweet
SLP05-> (RCA)-> Cary CAD200-> woofer
(Bi-amped). Result: CAD211AE works, CAD200 produces no sound from the woofer.

SLP05-> (RCA)-> Cary CAD211AE-> mid/tweet
SLP05-> (XLR)-> Cary CAD200-> woofer
(Bi-amped). Result: CAD211AE works, CAD200 produces no sound from woofer.

SLP05-> (XLR)-> Cary CAD200-> mid/tweet
SLP05-> (RCA)-> Cary CAD211AE-> woofer
(Bi-amped with valve amp on bottom). Result: Now here is the interesting thing. Put the SS amp on the mid/tweet and the valve amp on the woofer, and it works! I am succesfully bi-amping!

Now obviously this is not what I want, because I would rather have the SS amp on the woofer. I checked and rechecked the connections and there was no problem. I swapped RCA and XLR cables to my spares and there was no problem. I swapped speaker cables and there was no problem.

I even borrowed another two SS power amps and the result was the same - each time, the SS power amp refused to power the woofer in bi-amp configuration.

I am wondering whether there is something about the higher input impedance of the valve amp that makes the SLP-05 preferentially drive it.

This problem has me beat. I can't figure it out. Can anyone help?
amfibius
At a glance it looks like SLP05 to CAD200 via XLR is problematic when both amps are connected. Have you tried this combination?

SLP05-> (XLR)-> Cary CAD200-> woofer
SLP05-> (RCA)-> Cary CAD211AE-> mid/tweet
Never mind. Looks like you exhausted the combinations. I give up. :)
This is puzzling. I use both XLR and RCA outs from my SLP-05 simultaneously but for a different purpose - I use the XLR outs to full range hybrid power amp for my main speakers and the RCA outs to a SS AVR for rear surround speakers. This works fine. I would suggest you run it past Cary since you're running all-Cary electronics - they are likely to give you an accurate diagnosis.
SLP05-> (RCA)-> Cary CAD211AE-> mid/tweet
SLP05-> (XLR)-> Cary CAD200-> woofer
(Bi-amped). Result: CAD211AE works, CAD200 produces no sound from woofer.

SLP05-> (XLR)-> Cary CAD200-> mid/tweet
SLP05-> (RCA)-> Cary CAD211AE-> woofer
(Bi-amped with valve amp on bottom). Result: Now here is the interesting thing. Put the SS amp on the mid/tweet and the valve amp on the woofer, and it works! I am succesfully bi-amping!

... I even borrowed another two SS power amps and the result was the same - each time, the SS power amp refused to power the woofer in bi-amp configuration.

These are the key clues, it seems to me. And I think that they indicate that the preamp-to-power amp interface is not where the problem is; it is somehow in the power amp-to-woofer interface. The preamp would have no knowledge, so to speak, of which drivers (mid/hi or woofer) each power amp is connected to.

Are you sure that in the cases where you get sound from the woofers (running the ss amp full range, or running the tube amp into the woofer) you get the full expected volume from the woofers, distortion-free? Perhaps some problem has arisen in the woofer or its crossover elements that is causing the impedance presented to the power amp to drop down too low, causing the ss amps to go into protective shutdown, while the tube amp perhaps manages to keep functioning.

Or perhaps the woofer and its crossover elements by themselves present a highly reactive load to the power amp (probably inductive), and the ss power amp's protection circuitry is unhappy with that. When you run full-range, the capacitive reactance which is probably seen looking into the mid/hi terminals would to some extent cancel out the inductive reactance seen looking into the woofer terminals, resulting in a more purely resistive (and hence easier) load.

Regards,
-- Al
Thanks for your reply Almarg, I am finding it a bit difficult to follow your argument but what you say makes sense. To answer your question - every time I get sound from the woofers it sounds OK.

To clarify matters I am going to ask an engineer friend of mine to help measure the impedance curve of the woofers. I do not understand why the SS power amp is happy to power the speakers full range, but not happy to power the woofers alone.
To clarify matters I am going to ask an engineer friend of mine to help measure the impedance curve of the woofers. I do not understand why the SS power amp is happy to power the speakers full range, but not happy to power the woofers alone.

That sounds like an excellent idea. What I was speculating, and it's just a guess, is that the impedance looking into the woofer terminals alone is too highly inductive, such that it would trigger the ss amp's self-protection mechanisms. When you parallel the woofer and the mid/hi elements for full-range operation, the somewhat capacitive load which is probably presented by the mid/hi crossover elements would tend to partially cancel the inductive component of the impedance of the woofer and its crossover, making the overall load more purely resistive and therefore perhaps not triggering the amp's self-protection mechanisms.

Just a guess, as I say, but it's the only theory I can think of that seems to fit all of the facts.

Good luck!

-- Al
Also, if at all possible try to arrange for the impedance measurements to provide the phase angle of the impedance (vs. frequency), not just the magnitude of the impedance (vs. frequency). That will allow the inductive, capacitive, and resistive components of the impedance to be distinguished from one another.

Regards,
-- Al
Amfibius,

I have tried all the combinations you list with my Cary SLP-05, Cary MB 500 and Cary 120s tube amp and they all work for me.
I notice this statement in the manual for the CAD200:

Short circuit protection activates if load impedance is about 1.6 ohms or less.

I'm not sure how the protection circuit would sense what the load impedance is, but presumably it would do it by some sort of measurement of the relation between output voltage and output current. If the load is highly inductive, the current drawn by the speaker will lag the voltage by a substantial fraction of 90 degrees (exactly 90 degrees for a pure inductor, that has no resistance).

Therefore at the instant that current reaches its peak, the voltage will be at a considerably smaller value than the instantaneous voltage which caused that peak, which occurred somewhat earlier in the sine wave cycle. The protection circuit may interpret, however, that the peak current was caused by the instantaneous voltage occurring at the exact time of the current peak, in which case it would "think" that a lower impedance is present than is really the case (recall Ohm's Law, resistance = voltage/current; therefore a lower voltage producing the same current implies lower resistance).

When you run the speakers full range, as I said earlier, the capacitance which is likely to be a significant part of the impedance of the mid/hi drivers is placed in parallel with the woofer and its crossover elements, which would partially negate the inductive component of the loading presented by the woofer (the effects of inductance and capacitance on the phase relationships between voltage and current work in opposite directions).

So I think the statement in the manual about the protection mechanism reinforces the theory I presented earlier. It could be that the design of the amp's protection mechanisms is incompatible with your woofers, when they are driven alone. While you are waiting for the measurements you may want to ask Cary if a heavily inductive load, such as the woofer section of a speaker connected by itself, can trigger protective shutdown of the amp.

Regards,
-- Al
P.S: I just noticed in your system description that the woofer impedance "gets down to 2 ohms." I think that strongly reinforces the theory I have offered.

-- Al
Al's observation is probably right on the money, the problem looks like load impedance related than anything else. There aren't many amps that well behaves at 2 Ohms and below. You might want to audition First Watt F5, as it's supposed to handle down to 1 Ohm. It sounds awesome as well.
Thanks for your contributions so far, Al and others. Just one question - what would the symptoms of protective shutdown be? Would you expect the amp to make some sound, or would it refuse to power up altogether?

There are a couple of things which I did not mention in my initial post. The first is that I had to use 10m speaker cable, because I don't have a spare pair of anything shorter. When that failed to work, I hacked up some rather thin gauge lamp wire with the same results. I would imagine that both cables would present the amp with a large resistive load (and large capacitative load in the case of the 10m cable). Would this exacerbate the problem?

Also, I know something about the internal configuration of my speakers. It is rather weird - one woofer runs off the crossover, and the other woofer runs straight off the binding post and is effectively full range. What would this do to an SS amplifier?

Also, why does the SS amp refuse to drive the woofer, whilst the valve amp is perfectly happy to do so? It seems counter-intuitive.
Keith,
At some point I briefly tried the CAD 200 on the woofers of my Acapellas and had no issues at all. My pre is Supratek Dual Cabernet. The CAD 200 is a nice amp for the money, with midrange being an antithesis of typical solid state sound, but its bass is unfortunately slow and underdamped, the exact opposite of what your Violons need.
Hi Keith,

I would expect that the short-circuit protective shutdown, which as I quoted from the manual kicks in when it senses a 1.6 ohm load, would result in no sound whatsoever (aside perhaps for a fraction of a second or whatever amount of time it requires to activate). Also, fyi, the manual indicates that to recover from that condition the amp has to be shut off for at least 10 seconds, and then restarted. It also cautions that repeated protective shutdowns can degrade the output transistors.

I wouldn't think that there is much significance to the fact that you used long cables. The slightly increased resistance would work in the direction of reducing the likelihood of a protective shutdown, by increasing the value of the ohmic load (since the cables are in series with the woofer). What might hurt slightly is the increase in inductance (for the reasons I have previously stated), but the cable inductance is likely insignificant relative to the woofer and crossover inductance. Cable capacitance is likely a non-factor here.

In itself, the unusual configuration of the two woofers is not particularly significant. What is significant is that the overall woofer arrangement is highly inductive, the voicecoil of one woofer being directly in series with the amp output, and the voicecoil of the other woofer being in series with the amp output through (undoubtedly) an inductive crossover element.

The reason that the valve amp's ability to drive the woofers is counterintuitive is that from a SOUND QUALITY standpoint a tube amp, with its higher output impedance and more limited output current capability, is typically a poor match for a very low impedance speaker or driver. But here we are not dealing with a sound quality issue. We are dealing with an amplifier self-protection issue, and this particular amplifier has a protection mechanism that considers 1.6 ohms or less to be a short circuit. With the woofer impedance being around 2 ohms, and undoubtedly lower than that at some frequencies, sad to say that is simply an incompatible matchup.

The tube amp's higher output impedance, while degrading the sound quality it can provide into a low impedance load, would in fact help it to deal with a low impedance load, from a protection standpoint. The higher output impedance would in itself somewhat limit the current that would be drawn by a short or very low impedance, and provide some measure of protection that a solid state amp, with its much lower output impedance, would not have.

And for many other conceivable reasons the protection mechanisms of the tube amp, assuming it even has any, would figure to have little similarity to those of the solid state amp.

In any event, despite Kotjac's successful brief use of a CAD200 with the (apparently) same woofer configuration, the matchup of the CAD200's protection mechanism with the 2 ohm or so woofers is obviously extremely marginal at best. Any factor which would even slightly exacerbate the situation, such as the increase in net inductance which would result from driving the woofers in the absence of the capacitive loading presented by the mid/hi crossover elements, clearly stands a very good chance of putting the situation over the edge and triggering the self-protection.

The bottom line is I feel virtually certain that if you want to biamp, you need a different amp for the woofers. There is simply an inadequate margin between the woofer impedance and the short-circuit protection threshold.

Regards,
-- Al
Hey guys, I'm wanting to do the very same thing as I have 2 sets of monoblocks I'd like to use with my Cary SLP-05. Two CAD-211 FE's which I'd like to use for the mid/hi and Spectron Musician III Mk II fully balanced mono's. The Spectrons have no problem running down through 1 ohm, but the conceptual issue I have before even getting this set up (waiting for a rack for the Cary amp's so they're somewhat protected from the kids running around) is that the impedence matching (input impedance) is so different for both amps and that fully bridged the Spectrons will need to be attenuated (add to that, and this I would think would be relevant for everyone doing this, isn't XLR running about 6db louder than unbalanced RCA connections?). So my LF would be far too loud relative to the mids/highs unless I introduce a passive, fixed attenuator (say in a XLR tube at the amp input) spec'd to the right level so as to minimize quality degradation. Anyway, this is my chief concern, level matching.
Silverlight99,

I took a look at the following documents:

MusicianIII Specs
211FE Manual
SLP05 Manual
Stereophile's SLP-05 measurements, particularly output impedance

The 211FE input impedance appears to be specified as 150K unbalanced, 300 ohm balanced. If that is correct (and I would suggest verifying the 300 ohm number with Cary), you don't want to use the balanced input, because it's input impedance is vastly too low in relation to the preamp's output impedance.

Otherwise, I don't see any problems in relation to impedances. The power amp's 20K input impedance is not quite ideal in relation to the preamp's output impedance at 20 Hz of 3400 ohms (1500 ohms at higher frequencies), but that will result in a low frequency rolloff of less than 1db at 20Hz, which is insignificant.

You WILL need passive attenuation on the inputs to the Musician III's. They are spec'd for a voltage gain of 26db, apparently in stereo mode. Bridged mode will increase that to 32db. Using the balanced inputs may or may not increase that to 38db, depending on the internal design (you should ask Spectron that question).

The gain of the 211FE's is a little unclear. It says that 2 volts input results in full power, but full power is specified with 3 different numbers for Class A, Class AB2, and Class B operation. My guess, and it's just a guess, is that the 2 volts corresponds to the 110W Class AB2 number. For an 8 ohm speaker load (connected to the 8 ohm taps) that would be a voltage gain of about 24db (which would be 25db if the 2 volt sensitivity rating is based on the 150W Class B number). For a 4 ohm speaker load, those numbers would be about 20db and 22db, respectively.

So once you've pinned down the uncertainties I've described that enter into the gain calculations for each amp, the amount of attenuation that is required can be calculated by subtracting the resulting gain number for the 211FE from the gain number of the Musician III's.

All of this assumes that speaker sensitivity (spl out vs. voltage in) is the same for the low frequency drivers as for the mid/hi frequency drivers, which is probably the case if all drivers are part of the same speaker.

Hope that helps,
-- Al
Addendum to my previous post: I see on this spec sheet that the 211FE's balanced mode input impedance is listed as 300K, not 300 ohms, so the indication in the manual of 300 ohms is most likely a mistake, and using the balanced input is most likely fine.

Regards,
-- Al
Almarg - thx very much for the response, most helpful. slow delay as I didn't get a notifier for a response (don't know if that's a possibility on this forum). so I'm now getting closer to giving this a run. The speakers I'm using are the new eFicion F300's. they're an excellent speaker, with two separate units - bass and then mid/high. I'll check with the manufacturer if they're both rated at 8 ohms and how consistent. I will let you know what success I have! I'm guessing as a check if I set up the system without the attenuator on the bass from the Spectron's I can measure the actual voltage arriving at the speakers as a confirmation, but not sure.
Silverlight (and Almarg) I thought I should post a follow-up to this thread, given that it was me who asked the original question.

I have since bought a pair of JL Audio F110 subwoofers, and a Behringer CX2310 crossover to remove low bass from the main speakers. The result has been a substantial improvement in bass clarity up to the midbass, and improvement in dynamic headroom of the system. The cost is from loss of midrange and upper treble resolution, as well as an annoying hum. No doubt this is due to the poor quality of the Behringer. A Marchand is on the way.

I think a much better solution is to forget about bi-amping from the SLP-05, and buy an external crossover. It would be even better if it were possible to bypass the internal crossovers of your speaker and go active. The extra cost of doing so is negligible, since you already own two pairs of power amps.