Preamp to Main amp impedance matching...what's your ratio?

[wyn palmer]

No one matches impedance anymore for maximum power transfer between components. 600 ohms out to 600 ohms in was very common when telephone engineering principals were the norm. Connections could be very critical back then to get low distortion, good signal to noise and freedom crosstalk and linearity issues. By bridging outputs now days and hoping the back emf doesn't smear the sound. Yes with refinements and modern circuit design 10 X is very acceptable. 20 X a very minuscule better depending on the device doing the driving and the complexity of the load of the input of the amplifier or the next piece in the chain. Just be glad we don't have to deal with less than perfect input and output transformers and terminating resistors and the like any more. We are still dealing with pieces with very high output impedances driving much higher input impedance with tube stuff. Thats where pricey cable can come in to play to get the same performance SS owners get with every day reasonably priced cables.

I drive a Hovland HP100 tube amp into a Rogue M180 tube amp and into electrostatic speakers with a minimum impedance of c.0.9 ohms at c. 30kHz. No pricey cable is required- just short runs of the appropriate off the shelf, low inductance, low capacitance, low resistance, 10 gauge speaker wire for the speakers and quality shielded 3GHz video cable at c. 20pF/ft for the phono. What more can you ask for? What more can a pricey cable give you?
By the way, I measured the 10 gauge wire. Including connectors- quality banana plugs- a 6 foot length has 16mohms of DC resistance (that's milliohms...) 1.2uH inductance and 120pF capacitance (yes, I do have the calibrated gear to measure this). It's densely multistranded copper at the 10 gauge diameter excluding insulator.

 

[sqlsavior]

Depends what you mean by matching.

By matching, I meant 'to make compatible' rather than 'to make identical'. The object is not to maximize power transfer.

Actually, can we keep passives out of this thread, stick to solid state or tube for this discussion.

Certainly, but it is not the passiveness providing the compatibility benefit; it is the magnetic attenuation, which is also usable by active preamps.
I thought it might be relevant.

 

[Sonance'84]

Certainly, but it is not the passiveness providing the compatibility benefit; it is the magnetic attenuation, which is also usable by active preamps.
I thought it might be relevant.

That is relevant and I didn't mean to dismiss it, but was thinking it would take the discussion down the wrong path, as my original question was pretty basic.

I'd be interested in what active preamps use such attenuation and if they're in my price range. Feel free to PM me any models you know of that I can check out.

 

[wyn palmer]

That is relevant and I didn't mean to dismiss it, but was thinking it would take the discussion down the wrong path, as my original question was pretty basic.

I'd be interested in what active preamps use such attenuation and if they're in my price range. Feel free to PM me any models you know of that I can check out.

Magnetic attenuation i.e. a transformer- exactly what benefit do you think you'll be getting by adding one to an active output stage?
In effect you are trading current margin for voltage margin in the active buffer. For example if the transformer is 2:1- that is the output voltage is half the input voltage- then for the same voltage (and also current) into the power amp the active buffer will have to supply half the current, but twice the voltage. This may not be a good thing, as was shown in my example in post #20, and it also forces the stages prior to the buffer to operate at twice the voltage swing that they would have, potentially reducing margin. Yes, if this is after the volume control, then it may not be a problem, but it depends on the design of your system. Potentially it may be more or less a wash, but don't forget that transformers are intrinsically a bit lossy and in electronics as in life there ain't no free lunch...
One thing you might gain is an improvement in bandwidth if the active preamp output impedance reacting with the external capacitance is the main definer of the bandwidth. In this case the 2:1 ratio will effectively reduce the load capacitance by 4.
However, you also need to have a quality transformer in order to avoid limiting the LF bandwidth due to the input winding inductance which has to be large enough that it doesn't act as an attenuator with the output impedance of the buffer. If the active buffer has a low output impedance then it's less of an issue, but if that were the case then we wouldn't be considering this.
And this is by no means the end of the potential consequences...
Once again, this was written quickly and without any checking, so I apologize for any errors that I might have made.

 

[David_NC]

Pre = before.

The "amp" in pre-amp is referring to the main amp, it dose not mean that the "Pre" is an amplifier.

Active pre-amps are not amplifying anything, >>most of the time<<, they are attenuating.

Passive pre's are before the main amp so "Pre-Amp" (Before-Amp) is correct.

I'm wondering how a phono pre-amp works...

Phono > pre-pre-amp (aka phono pre-amp) > pre-amp > power amp?

I always thought pre-amp was the amp before the amp that powers the speakers, and the amp that powers the speakers is the power-amp. I have a couple integrateds and receivers that have pre-amp out / main amp in... shouldn't they be labeled line-level controller out / main-amp in? LOL All my pre-amps DO amplifiy; they have built-in phono sections which definitely boost / amplify the signal. I have an old Sony TA-2000 with tape head inputs rated at 1.2mV max (same as the phono imput but without euqlization), but the TA-2000's main outputs are 1-2.5V, but the tape outputs are 120mV. I'd think that both 'what comes before the main amp' and 'the amp before main amp' both work... at least sometimes!

 

[Sonance'84]

Magnetic attenuation i.e. a transformer- exactly what benefit do you think you'll be getting by adding one to an active output stage?

Are you asking me or the person I was quoting? If you're asking me...I have no idea. I said I'd be interested in learning more, via PM, as not to have this thread sidetracked like this. While very informative, it has nothing to do with the simple question I originally asked. Your contribution to this thread is helpful, no doubt, but I was just asking for what rule of thumb people use when pairing preamps with power amps. I don't know what all this speculation about passive preamps has to do with my original question?

 

[restorer-john]

I understand what you are saying - I think -grammatically . . . .

But friend, these terms have been in vogue for ever since the Williamson amplifier emerged about 1946 and well before, and have since been accepted over many decades dealing with high-fidelity equipment and its associated terminology, to mean certain things. All used in countless books, all taught in university and technicon syllabi ???

A pre-amplifier has always described an amplifier used to amplify-and-control/filter/whatever a signal prior to being fed to a main or power amplifier. When signal sources started to deliver higher signal amplitudes e.g. from tuners, CD-players, digital sources and the like, amplification got lower and controls fewer, often down to a volume control only.
In the hey-day one required quite a degree of amplification between a record pick-up and a power amplifier apart from the classic equaliser. The term originated from 'amplifier-before-a-power (main) amplifier' and yes, it does amplify. In the days of tube amplifiers pre-amplifiers delivered signals of the order of 100mV to about 2 V (all r.m.s.) to a power amplifier. Even the term 'power amplifier' is in a sense misleading - no power as such is amplified - signal voltage is amplified, and only when sufficiently high converted into power, usually during a single final stage to drive a loudspeaker.

Etc. etc. etc. etc. ad infinitum.

So kindly pardon if I have some difficulty to understand why you are now, at this point in time after nigh a century, trying to introduce fresh terminology? I would respectfully suggest that you consult the internet or classic reference works on the use of common amplifier terminology.

Yes, accuracy is often sacrificed in favour of descriptive terms - how scientifically accurate (or grammatically if you wish) - is the term 'push-pull ? Yet all audio folks know what it means - and try to change it at this time!

Actually, respectfully, you have me at a disadvantage as to why I should even have to explain all this to you? As often said: 'The internet is your friend' - and I would advise starting at audio basics.

Well said.

 

[david1111]

10x is fine, but I like to have a pre with a low output impedance cuz the signal is more intact thru longer ICs to the main amp. (I used 'main' instead of 'power', to avoid unwanted attention from Wordies). I currently have my amp between my speakers.

Dave

 

[edwyun]

IME, an impedance ratio of 10x may work. But if you want better bass performance/bass slam, you should look at an impedance ratio a lot more than 10x. Proof will be in the listening.

 

[wyn palmer]

IME, an impedance ratio of 10x may work. But if you want better bass performance/bass slam, you should look at an impedance ratio a lot more than 10x. Proof will be in the listening.

Why? What possible reason would there be for this to be true?

 

[wyn palmer]

10x is fine, but I like to have a pre with a low output impedance cuz the signal is more intact thru longer ICs to the main amp. (I used 'main' instead of 'power', to avoid unwanted attention from Wordies). I currently have my amp between my speakers.

Dave

Lowering the output impedance arbitrarily to drive capacitive loads should be done with some caution as the output impedance of buffer stages with negative feedback rises with increasing frequency and hence looks like an inductor. This reduces the phase margin of the buffering amplifier and it can also resonate with the capacitive load producing peaks in the frequency response or even oscillation. It's better to have some finite real part to the output impedance so it's often beneficial to have an actual resistor in series- anywhere from 1ohm to 1kohm depending on various factors.

 

[rothwellaudio]

It's better to have some finite real part to the output impedance so it's often beneficial to have an actual resistor in series- anywhere from 1ohm to 1kohm depending on various factors.

Yes, I use 200 ohms as my default "that'll do the job" resistor for the reasons you state.
BTW, where output coupling capacitors are used I think they're often too small, particularly with DIY designs, because the builder wants one of those physically very large capacitors from an audiophile vendor which costs a lot of money for not very many microfarads.

 

[Audiovet]

I didn't ask you to explain anything to me, I don't need any explanation, I already know what "pre-amp" means and I stand by what I said.
Having said that, this is hardly worth arguing over, I was just trying to clear up a common misunderstanding on what Pre-amp stands for. We will have to agree to disagree.

Oh dear ..... and no - we don't!

Perhaps (or perhaps I should say 'definitely') this can be ended by referring all these knowledgeable gentlemen to 'pre-amplifier' ('Wikipedia') or any other recognized textbook/publication on audio amplification.
We can all coin our own definitions, and I daresay some will be more to the point than many of the accepted parlance.

Still

Also from me to the OP: Apology for deviating; I was not expecting this - never mind . . . .

 

[david1111]

Lowering the output impedance arbitrarily to drive capacitive loads should be done with some caution as the output impedance of buffer stages with negative feedback rises with increasing frequency and hence looks like an inductor. This reduces the phase margin of the buffering amplifier and it can also resonate with the capacitive load producing peaks in the frequency response or even oscillation. It's better to have some finite real part to the output impedance so it's often beneficial to have an actual resistor in series- anywhere from 1ohm to 1kohm depending on various factors.

Well, I don't know if I'm being arbitrary about it, or not, but my CJ PV-5 pre has an output impedance of 200 ohms, and my former tube pre had a 2000 ohm impedance. I've generally heard that 200 to 600 ohms is sort of a desirable output impedance range for a preamp.
I certainly have less issues with the 200 ohms vs the 2 kohm output.

Dave

 

[david1111]

@wyn palmer
Is there a possibility that the 200 ohm output of my Conrad Johnson is too low? Llike they may have made it to mate with a CJ mosfet amp?
Thanks for the info.

Dave

 

[Pio1980]

@wyn palmer
Is there a possibility that the 200 ohm output of my Conrad Johnson is too low? Llike they may have made it to mate with a CJ mosfet amp?
Thanks for the info.

Dave

No, not really. Low is good, it helps negate cable and other loading issues. Many CJ preamp line stages are simple cathode follower or similar line drive, and may invert acoustic polarity. See instruction manual for the specific model for particulars.

 

[david1111]

No, not really. Low is good, it helps negate cable and other loading issues. Many CJ preamp line stages are simple cathode follower or similar line drive, and may invert acoustic polarity. See instruction manual for the specific model for particulars.

Hi Pio. They do tell you in the manual to try reversing the A/C plug to see if the sound improves.

Dave

 

[Pio1980]

As I found with my CJ Motif FET line preamp. Definitely made a difference in my case.

 

[david1111]

Yeah, for me too.

Dave

 

[Denten]

Oh dear ..... and no - we don't!

Perhaps (or perhaps I should say 'definitely') this can be ended by referring all these knowledgeable gentlemen to 'pre-amplifier' ('Wikipedia') or any other recognized textbook/publication on audio amplification.
We can all coin our own definitions, and I daresay some will be more to the point than many of the accepted parlance.

Still

Also from me to the OP: Apology for deviating; I was not expecting this - never mind . . . .

First I would like to apologize for this, I'm not known around here for pissing people off and I regret posting that here, I was not trying to start a argument.
I agree that pre-amp stands for pre-amplifying a audio signal before the main amp. Even Luxman calls there new line of pre-amps Control Amplifiers.

In response to this:

(Again one of the many misnomers in this field; they should be called 'control units' since there is no amplification.)

I was trying to show that "Pre-Amp" can make sense for passives too, it can work for both. I was not trying to change the terminology, just how it is interpreted when referring to passives.
I respect your knowledge and I could learn a lot from you.

Respectfully Tracy.