Load for FET ten hl

[Lavcat]

I currently run my FET ten into a 10k ohm load. I'd been planning to convert to using a 10k:10k transformer. Yesterday I had the thought to possibly using a 600:600 transformer instead.

The output of the FET ten is specified as 100 ohms in series with 100 uF. Distortion of the FET ten is characterized into a 10k load.

Modeling the circuit in SPICE, the FET ten output into a 600 ohm load at 20 Hz was down 0.05 dB. Assuming of course that I made no errors. Response was better into a 10k load, however.

If I can live with 0.05 dB rolloff, is there anything wrong with using the FET ten with a 600 ohm load?

[Zen Mod]

just go for it , but let your ears decide about difference , not sym program

[Lavcat]

SPICE was free, two pair of transformers are not. Makes for an expensive test. When I get my tax refund I guess I'll call Jensen and discuss the circuit.

I am thinking to do something like the Jensen app note as060, but my load is lower than specified, that is 20k not 40k. I could change R1 and R2 to 10k up from 4.99k, but that would form a bridge with each arm equal to 10k. I understand that for best common mode rejection the source side of the bridge should be lower than the output side.

If I used a 600:600 transformer then the resistors would end up being 309 ohm or there abouts, rather than 10k.

I was hoping someone could tell me if there was any other reason not to use a 600 ohm load on the FET ten besides the slight bass rolloff. But, I agree, in an ideal world I would get some of each transformer and listen for what sounds best.


http://www.jensen-transformers.com/as/as060.pdf

[Zen Mod]

what's important is ratio , not so called impedance ;

impedance of load transformed from sec to primary is strictly in function of load itself , and it will be exactly the same in both cases , because both examples are 1:1 transformation wise

what's important is - does "600 Ohms" xformer have enough inductance in primary , to give low enough F3 , while having enough extension in "right side " of spectrum .

you can always have two extreme types of xformers of same "impedance" - one targeting at low and one targeting at high inductance ;
in case of one with low L, you'll need loading of secondary to achieve enough low frequencies (while choking high ones ) and with highish L - you'll need to load sec, to ensure enough high frequencies (while choking low ones with loading ) .

if you're lucky - you'll get one with medium inductance , needing none or just slight loading

so - for safe bet - just buy 5K or 10K one

catch22 is that you can sim xformer only if factory gives you exact model , which will give you precise results ........ and that's hardly the case

unfortunately - I can't pick schematic now from my main PC and I'm lazy to search for it , so I can't see what's output impedance of preamp as is , and what can be in case of any possible simple revision ....

[Lavcat]

Originally Posted by Zen Mod

what's important is ratio , not so called impedance ;

impedance of load transformed from sec to primary is strictly in function of load itself , and it will be exactly the same in both cases , because both examples are 1:1 transformation wise

I think we both know how a transformer works. I'm using transformers for galvanic isolation and balancing, not for impedance matching. Let me simplify the question: should the FET ten be unhappy driving a 600 ohm resistive load?

As an experiment I have been driving a pair of 600 ohm 'phones (Koss Pro 600a) on the output of the FET ten. The bass is somewhat anemic but I think that's just the Koss. They sound about the same driven from a low impedance amp as from the Threshold.

But I expect I will probably end up using the 10k:10k transformers rather than 600:600 transformers. I have a couple of them and I know they work well on the input of the FET ten. Cheaper too. An even better reason is I just noticed the 600:600 model would not fit in my enclosure.

[Nelson Pass]

Originally Posted by Lavcat

Let me simplify the question: should the FET ten be unhappy driving a 600 ohm resistive load?

It was designed to drive 600 ohms.

The thing to remember is that the transformer input impedance is also a
function of the impedance that it is driving. If you take a 600:600 ohm
transformer and load it with a much higher impedance you will often get
something like 6K ohms input impedance, and this will vary somewhat with
frequency.