can tz102 drive my amps?

Klas · « **on:** 15 Apr 2003, 08:49 pm »

i know i should be able to figure this out, but it really isn't clear to my novice mind.

my power amps include input step-up transformers, 500ohm:15k.
is the 500 ohm primary impedance too small for the tx102's to drive?

any help is greatly appreciated

thanks,

- k

John Chapman · « **Reply #1 on:** 15 Apr 2003, 09:37 pm »

Hello!

Good question! Also one I can't answer right off. It looks from the specs you give that you have a transformer that is about a 1:5.5 ratio and so it would transform a 15K impendance on the secondary back to a 500 ohm impedance on the primary (that the TX102 would see). The impedance changes by the square of the stepup ratio (5.5 * 5.5 = 30 - or close enough). This does not mean it has a 500 ohm input impedance for sure. If the connected secondary impedance were higher than 15K then the input imp to the trannie on your amp would be higher as well. I think the inductance of the primary plays a role in this as well.

I would be very surprised if your amp really did have such a low input impedance as it is a very low figure - much lower than any I have seen before. So.... Can you point me to any more info on the amp? We'll dig into it and figure it out.

Many Thanks!

John Chapman
www.bentaudio.com

Klas · « **Reply #2 on:** 15 Apr 2003, 10:43 pm »

the amp in question is homemade and home-designed.
you are correct about the turns ratio.
the top of the input step-up transformer secondary is connected directly
to the grid of a 71a. the bottom is connected to ground.
the primary is connected to the input jack and is completely isolated and floating from ground.
at present there is no resistor across the secondary.
i expect to experiment with different values of resistors at this location though.
the transformer is actually a linestage output transformer, connected as a step-up input, and i have been driving this amp with an active linestage that uses the same transformer on it's output. so in effect i have the linestage and the power amp coupled via two identical transformers, mirroring each other.
nevertheless, the amps just sound so much better when i omit the active linestage altogether, and i don't need the gain, so i want to try passive.
And I think the tx102 might be the way to go.
thanks for your help.

best,

- k

Marbles · « **Reply #3 on:** 15 Apr 2003, 11:40 pm »

I don't understand much of the above, but my amps have an input impedence of 10K Ohms and the silver wound TX102 drives them very nicely....

John Chapman · « **Reply #4 on:** 16 Apr 2003, 01:13 am »

Hello!

We usually figure about 10K as the low end of the preferable range for input impedance and most amps are higher than this. Thanks Marbles for letting me know that you are at the 10K point. I can measure the TX102 here and it does well into low impedance loads (provided the source also has a lowish impedance to drive the TX102/Amp combo - which is usually the case). The amps I have around here tend to be a bit higher than that in input impedance so I have never had the chance to play with low input impedance amps here myself.

The TX102 at typical levels has an output imedance lower than the source's impedance. At low levels (when it is turned down) the output impedance is much lower than the source's output impedance. If I have understood your playing around there you have directly connected the source to the amps and then had a listen to it. If this is the case (and I take it that it sounded good!) then your source will be fine to drive the TX102 feeding the amp. As you lower the level of the TX102 the output impedance will drop even more - providing more drive to your amp.

If I have guessed right about the connection you have made then could you let us know if the level when directly connected was as loud (or louder) than you would ever care to listen. If this is the case and you did not need to go louder then this then the TX102 should work fine as a level control placed in there.

Many Thanks!

John Chapman
www.bentaudio.com

Klas · « **Reply #5 on:** 16 Apr 2003, 11:36 pm »

hi john,

You are correct. In my system, connecting the sources to the power amp directly is plenty loud. Also, the output impedance of my sources are low enough to drive the tx102, i think. DAC is 600 ohm and phono stage is 7k. So, the last question for me is, what is the input impedance of my power amps. I wish I knew. Here is a schematic of a different amp, but the input stage is very similar to mine. Anyone know how to calculate the input impedance for this type of circuit?
http://www10.big.or.jp/~dh/work/78101.html

thanks again.

- k

John Chapman · « **Reply #6 on:** 18 Apr 2003, 03:25 pm »

Hello!

Sorry for the late reply! I am a bit of a hacker when it comes to tube amp design so I was hoping one of the experts might chime in. I do know the grid resistance would be seriously high and even with the 30:1 impedance transformation I'd still think the input imp would be pretty high. The fact that your sources seem to work well plugged directly is an indication that the input impedance must be not too low for them to drive. The 7K output impedance of the phono stage is quite high - are you sure that is the figure and did you have it directly connected as well? If you did then your amps input impedance must be decently high if that 7K output imp phono amp was able to drive it well.

Thanks!

John

Klas · « **Reply #7 on:** 23 Apr 2003, 02:51 am »

John, thanks for following up. I eventually found the answer to my question. If anyone is curious:
http://www.audioasylum.com/forums/bottlehead/messages/63535.html
Looks like it'll work out.

best,

- k

John Chapman · « **Reply #8 on:** 23 Apr 2003, 12:45 pm »

Hello!

Glad you got the scoop. Kurt is one of the experts I hope would be able to help!

Thanks!

John