Tír Na HiFi

I'd not recommend a direct connection of a trafo to the ladder output due to the high source impedance. But if you absolutely must then the 15k is going to be the best as that'll have highest shunt inductance. But its a step-down 15k:600 will give you SQRT(15000/600)= 5:1 voltage reduction. Can you live with that? If not then 10k:10k 1:1 is your next best bet but that's gotta suck, seriously....

ok
need to determine what is wimpy impedance <1k?
what is nominal >=10k

so impedance matching is essentially a load of bullshit
and what needs to be done is to transfer the signal without distortion by over loading the source's output impedance


for this app the transformer input impedance must be much less than the source impedance?

so when i put the soekris's z=625 output through a 25k pot then into opamp the sound was all over the place (aka a terrible mess)
this was because the soekris output just cant handle the load?
when i put a b1 type buffer between the pot and amp things improved substantially
in hindsight this pot should be after the buffer?
and ive seriously compromised the sound putting a pot on the soekris output?

the reason the buffer was in the amp and not the dac was/is because a suitable buffer ps is already in the amp

so ideally what needs to be done is soekris-buffer-transformer-amp-tvc-speakers?
and soekris-buffer-transformer-potentiometer-amp-2:0 opt-speaker might not suck too bad?

nige2000 wrote: need to determine what is wimpy impedance <1k?

'Wimpy' isn't just about impedance, though that's most of it. An R2R DAC is going to be sensitive to loading on its output so that makes it even more wimpy than if it were a beefy buffer with a 625ohm series resistor. I'd say wimpy was higher than 200ohm source impedance myself, but its a bit arbitrary.

what is nominal >=10k

Yeah 10k is the bare minimum input impedance, 47k is respectable and 100k and above is excellent.

so impedance matching is essentially a load of bullshit

In the context of audio interconnections as far as I can see, yep its a bunch of baloney. In the RF world though, its mandatory because matching means no reflections. So it becomes important for an S/PDIF link and very important for HDMI.

and what needs to be done is to transfer the signal without distortion by over loading the source's output impedance

That's one aspect but another is cable capacitance mustn't roll off the response which is what happens with too high a source impedance. Also the load impedance shouldn't vary (as it does with transformers for example).

for this app the transformer input impedance must be much less than the source impedance?

I guess you meant the opposite - much higher than the source so as not to affect it much.

so when i put the soekris's z=625 output through a 25k pot then into opamp the sound was all over the place (aka a terrible mess)
this was because the soekris output just cant handle the load?

That would be my guess yep. Too sensitive to loading.

when i put a b1 type buffer between the pot and amp things improved substantially
in hindsight this pot should be after the buffer?

I'd say so yes.

and ive seriously compromised the sound putting a pot on the soekris output?

Trust your own ears.

so ideally what needs to be done is soekris-buffer-transformer-amp-tvc-speakers?

Yes that would be my ideal set-up.

and soekris-buffer-transformer-potentiometer-amp-2:0 opt-speaker might not suck too bad?

Not sure, I'd want to have a higher impedance pot than 25k if possible. It will depend on the input impedance of your amp.

well the amp is audiosectors lm3875 and its got a signal 22k to gnd

think ill look into amp output tvc (to ditch the potentiometer), what kind of switch would be suitable for the tvc

think ive one of these somewhere


http://www.ebay.com/itm/1PC-2-Pole-23-S ... rkt%3D1%26

with taking out the pot is 10k/10k most optimal? between buffer and lm3875?

nige2000 wrote:well the amp is audiosectors lm3875 and its got a signal 22k to gnd

That 22k I think is just tradition, it can go higher at the expense of raising the output offset voltage. With 100k the typical offset would be 20mV.

think ill look into amp output tvc (to ditch the potentiometer), what kind of switch would be suitable for the tvc

Not sure, you'll want something that can safely handle 2 or 3amps.

with taking out the pot is 10k/10k most optimal? between buffer and lm3875?

You'll want to manage the gain structure. A chipamp normally has about 26dB of gain, your supplies are 32V so 1VRMS will be your input sensitivity. What's the Soekris output level max?

abraxalito wrote:
You'll want to manage the gain structure. A chipamp normally has about 26dB of gain, your supplies are 32V so 1VRMS will be your input sensitivity. What's the Soekris output level max?

well vref is supposed to be 4 volts and 1.4vrms in spec sheet, i have vref at 3.3v so its a little less i think

So then you need no attenuation at all between the Soekris and the LM3875. On quiet tracks you might even want digital gain.

As is 10v is moderate listening level for vast majority of stuff
Likely never go above 15v out

nige2000 wrote:As is 10v is moderate listening level for vast majority of stuff
Likely never go above 15v out

As you never go above 15V to your speakers (I take it that's what you mean here) then the 2:1 trafos are perfect and will optimize the use of the amps. You'll need 6dB extra gain with the trafos in. With TVCs in you also will want 6dB more gain to be running the amps closer to the rails. ClassAB amps get more efficient at higher levels (like any amp) so you make better use of your power supply.

You'll need 6dB extra gain with the trafos in. With TVCs in you also will want 6dB more gain to be running the amps closer to the rails. ClassAB amps get more efficient at higher levels (like any amp) so you make better use of your power supply.

there is a 6db loss using opt?
how do i get it back?

is this the correct concept of tvc for amp output