DECWARE.....ZTPRE anyone have any expeiriance with one ?

almarg
I see that the output tubes are coupled to the output connectors via capacitors having what to me is a very surprisingly low value of 0.1 uf

I just downloaded the latest I believe circuit.
And what Al has pointed out is quite bad, as if that preamp’s xlr output sees an input load of 20kohm the bass will be rolled of at -3db already at 80hz!!!! In other words you’ll have no low bass.

If it sees a Class-D amp which many have 10kohm input impedance, the bass will be -3db at 160hZ!!!! no upper bass either!!!

It’s rca output "could" be fine as they are isolated by the transformer coupling from outside amp impedance’s.

I suggest you don’t use the XLR output with any amp/s less than 100kohm input impedance.

Cheers George

Thanks, George. But actually it isn’t that bad. Note the presence of the 100K output level control. The cap will only "see" the input impedance of the power amp when the 100K output level control is at or near max. (Actually in that case it would see a heavier load than that input impedance since it would be paralleled with the 100K). And my suspicion is that under typical conditions the control would be set closer to the bottom of its range than to the top of its range.

So the resistance that forms a high pass filter in conjunction with the 0.1 uf would probably be comprised of the majority of the 100K, plus the parallel combination of the rest of the 100K and the input impedance of the amp. And if the control is used near the bottom of its range, that would amount to close to 100K overall.

Nevertheless, assuming the schematic reflects the present design, per some calculations I did prior to my earlier posts there would be upwards of 2 db of rolloff at 20 Hz even with no power amp connected and regardless of the setting of the control. Which already considerably exceeds the +/- 0.7 db tolerance of the 20 Hz to 20 kHz frequency response spec. There would also be phase shifts in the bottom octave that could very well be audibly significant. With the rolloff and the phase shifts especially likely to be problematical in a case like the OP’s, involving large speakers having excellent deep bass extension, and also for others who may be using subwoofers. (Compensating for the rolloff with the level control of a subwoofer would be a crude approximation at best, since both the amount of rolloff and the corresponding phase shift would vary as a function of frequency).

Best regards,
-- Al

almarg
The cap will only "see" the input impedance of the power amp when the 100K output level control is at or near max.

You sure Al, I see it like the cap seeing the "parallel" combination of what the volume control and poweramp impedance is combined, just like two paralleled resistors. As the pot goes to ground like the input of the poweramp would.

EG:
Pot = 100kohm
Amp=100kohm
Cap sees= 50kohm


Cheers George

Gentlemen....I may just bite the bullet and purchase both the Prima Luna and the ZTPRE....cut heads for 2 weeks and return one. I do have access
to Clio and could perform an RTA  to really check that bottom end on both.I am checking into PM's return policy now.

I really do enjoy these types of discussions as I can get so many educated points of view...then again that's why we all love this "hobby" (addiction, habit, relapsing disorder) so much :}
Have a great day,

Kevin

Hi George,

Yes, I’m pretty certain that what I said is correct.

The example you cited, resulting in the cap seeing 50K, applies if the pot is set at max. But since the amplifier with the 100K input impedance is connected to the wiper of the pot consider what would happen if the pot were set, for example, such that 10% of the pot’s resistance is between the wiper and ground, and 90% of the pot’s resistance is between the wiper and the point where the pot and the cap are connected.

With that setting the resistance between the point where the cap and the pot are connected together and ground (i.e., what the cap sees), assuming the same 100K input impedance of the amplifier, would be:

Cap sees: 90K + (10K in parallel with 100K) = 99.1K, not 50K

The rolloff of the resulting high pass filter in that example will be determined by the combination of 0.1 uf and 99.1K. The rolloff (and the corresponding phase shift) will occur at the point where the cap and the pot are connected together, and it will also occur at the wiper of the pot which in turn is wired to the output connector. The signal at the output connector and the signal at the point where the cap and pot are connected together will differ in terms of amplitude, of course, but not in terms of frequency response (putting aside the possible effects of cable capacitance and amplifier input capacitance on high frequencies).

Finally, to be sure we are looking at the same schematic, I am looking at the one shown on the last page of the manual.

Best regards,
-- Al