Preamp / amp impedance matching question

thedudeabides · Feb 11, 2014

Hi all,

Not a techie when it comes to electronics so I would appreciate anyone's input regarding a potential performance issue.

I have a Conrad Johnson ET5 tube pre with two pairs of single ended outputs. CJ's recommendation is to have amplifiers that have a minimum 20K ohm input impedance.

My Pass Labs X250.5 amp has an input impedance of 12K ohms.

Am buying the YG II Kipod Signature speaker / active version. Kipod bass amp has an input impedance of 10K ohms.

When I hooked up the ET5 several months ago, I noticed I had to increase the volume setting by about 3.5db to match the volume level I was getting with my previous CJ 18LS SS pre. I assume that was because of the impedance matching between the Pass amp and the CJ tube pre. Is that correct?

Now that I will be using both pre outputs connecting to amps that are below the recommended CJ input impedance values, should I be concerned?

Also, any special considerations for the interconnect between the pre and the Kipod other than low resistance? Will be an 18' run.

If you all can answer in non-tech, practical terms, I would be most appreciative.

Thanks in advance for any comments.

GG

puroagave · Feb 11, 2014

the output impedance of the ET5 is fairly low at 100 ohm, you should be ok driving both outputs even into the low input impedance the Pass/kipod amp presents. I don't know why the CJ 18 would have more gain since its output impedance is higher and swings less volts than the ET5.

microstrip · Feb 11, 2014

puroagave said:
the output impedance of the CT5 is fairly low at 100 ohm, you should be ok driving both outputs even into the low input impedance the Pass/kipod amp presents. I don't know why the CJ 18 would have more gain since its output impedance is higher and swings less volts than the CT5.

I think you are referring to the ET5 - the CT5 has high output impedance - nominal output impedance 800 ohm rising to 1836 ohms at 20Hz, we can guess that the preamplifier is capacitor coupled. Are you sure that the ET5 is not capacitor coupled?
The 18LS also has high output impedance.

puroagave · Feb 11, 2014

microstrip said:
I think you are referring to the ET5 - the CT5 has high output impedance - nominal output impedance 800 ohm rising to 1836 ohms at 20Hz, we can guess that the preamplifier is capacitor coupled. Are you sure that the ET5 is not capacitor coupled?
The 18LS also has high output impedance.

ok fixed.

the ET5 is a hybrid with a FET output, I don't know if the output is capacitor coupled.

DonH50 · Feb 11, 2014

I cannot comment on your specific components.

My old ARC SP3a1 fared poorly, measurements and sound, driving a 10k load. It was much happier with 50k - 100k. I found similar results with several other tube preamps back in the 80's. Before getting hi-Z input (tube) power amps, I actually added 10k series resistors so the preamp saw a 20k load and distortion was much lower though noise a little higher. It was long ago so I do not remember details, but I suspect a lighter load on the preamp might make it sound better. I did run a FET-input buffer stage for a while and that also helped, albeit at the cost of a little higher noise floor. Back then the noise floor was set by my LPs and tapes so preamp noise did not matter to me.

zermatt · Feb 11, 2014

The CT5 is capacitor coupled and you could have trouble driving both the bass amp of the speaker and the mid range/treble amps. If you hear a low level hum in the both speakers then then the pre-amp is driving both amps correctly. The solution would be to purchase a xover that has a 50K ohm impedance and that would be above the 20K limit of the CJ unit. I had this issue with my system.

thedudeabides · Feb 11, 2014

Thanks for the comments to date.

First off, I have the ET5, not the CT5.

I dug into the technical weeds a bit and here's what I found.

Several sources indicate that a minimum 10 : 1 ratio (amp input impedance divided by preamp output impedance) is a good rule of thumb. Others say the higher the ratio, the better. Upwards of 100 : 1.

In my case, the Pass Labs ratio would be 120 : 1 and the Kipod bass amp 100 : 1.

Just got off the phone with the CJ tech department.

Confirmed that there should be no adverse performance issues, within the context of impedance matching, with my proposed setup.

Thanks again.

GG

DonH50 · Feb 11, 2014

Are the bass and main amps going to be in parallel on the preamp's outputs? That's a 5.45 k-ohm load... Probably OK if there is a FET output buffer in your preamp as posts above indicate.

CJ should certainly know!

thedudeabides · Feb 12, 2014

Don,

Don't know if this answers your question but I did ask if each pair of preamp outputs had an output impedance of 100 ohms.

The answer was yes.

And it doesn't vary with frequency response.

GG

microstrip · Feb 12, 2014

thedudeabides said:
Don,

Don't know if this answers your question but I did ask if each pair of preamp outputs had an output impedance of 100 ohms.

The answer was yes.

And it doesn't vary with frequency response.

GG

I can not understand how it doesn't vary with frequency - cj refers that "the same CJD Teflon capacitors that distinguish the GAT are used in the ET5 for output coupling and post-regulator power supply storage." May be they are large enough to bring the cutoff to the infrasonic zone, even with a 5kohm load.

DonH50 · Feb 12, 2014

Hmmm... I assumed if the ET5 had a FET buffer it was direct-coupled to the outputs, and likely has a small series R for short-circuit protection (many preamps do). If there are coupling capacitors, then e.g. 1 uF is ~16 k-ohms at 10 Hz. The only thing I can think is the coupling capacitor is between the final tube stage and the output FET driver/buffer. That way the output buffer could be a pretty constant 100-ohm source.

thedudeabides · Feb 12, 2014

Don,

I believe that is "basically" how it works.

GG

thedudeabides · Feb 12, 2014

microstrip said:
I can not understand how it doesn't vary with frequency - cj refers that "the same CJD Teflon capacitors that distinguish the GAT are used in the ET5 for output coupling and post-regulator power supply storage." May be they are large enough to bring the cutoff to the infrasonic zone, even with a 5kohm load.

I specifically asked the question and that was the answer from the tech department. There was no hesitation.

I assume they know what they are talking about.

As I said before, I'm not a techie.

GG

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Preamp / amp impedance matching question

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