I ordered 2 before I thought about a group purchase, had other stuff in cart so I got free shipping, seems like a good deal.
So, we just got the 20th pre-order, so that's the end of the free shipping. Thanks everyone.
My only concern with the PE trafo is the neg rail voltage. At 16.2V, you will get about 22.7V recitifed, but the drop across the diodes/FETs in the power supply is about 4.5V, leaving you with a 18.2V neg rail.
I may pick a could of those to have for other projects anyway...
My only concern with the PE trafo is the neg rail voltage. At 16.2V, you will get about 22.7V recitifed, but the drop across the diodes/FETs in the power supply is about 4.5V, leaving you with a 18.2V neg rail.
I may pick a could of those to have for other projects anyway...
Even though the free shipping is over, are you just ordering enough parts for 20 kits or parts for a lot more kits? I'm still waiting to see what's the decision on the transformer spec and where to order the transformer from, before placing my kit order.
BrianDonegan said:
Hi Brian,
Are you sure you will lose 4.5V across the diodes? That seems high to me.
How critical is it to have +70/-20?
Thanks, Terry
You'll lose ~4V in the regulator, another .6 in the rectifiers.
Having less than 20V on the negative rail will reduce the negative voltage swing, but you should have more than enough to drive any reasonable amp with 18V on the negative rail. Kari's design point was 9V negative rails, IIRC. I haven't had a chance to build up mine yet, but haven't heard any complaints.
If you're worried about the rails, switch to an MJE1503X device and you'll only lose a volt or so in the regulator.
Having less than 20V on the negative rail will reduce the negative voltage swing, but you should have more than enough to drive any reasonable amp with 18V on the negative rail. Kari's design point was 9V negative rails, IIRC. I haven't had a chance to build up mine yet, but haven't heard any complaints.
If you're worried about the rails, switch to an MJE1503X device and you'll only lose a volt or so in the regulator.
still4given said:
Terry,
I think the 4.5V loss includes the VGS voltage of the
mosfet.
I don't think the 70/20 voltage is critical. Other
combinations will also work.
Just my $0.02 CAD.
Cheers,
Dennis
I have ordered parts for 25 kits (more for some parts). I have to balance risk/cash. I will likely order another 25 kits worth soon, so there is little risk of running dry. I do have some parts backordered, which is my only concern, but I am pretty sure they will be in before shipping. If not, I have backups for each.
As for the transformer, we are still waiting for some feedback, but since tend to weigh a lot, it's likely we won't be redistributing them. The PE trafo will probably work fine, it was just my hesitation. I may go with two small 18-0-0-18 Amveco trafos (from DigiKey), but I haven't decided yet either. It depends on what we hear back from some of the quotes we have asked for. It would be possible to ship those though, as they are pretty small.
BTW, for the FET matching...
I have 300 IRF610s on the way (25 kits worth). My plan is to match sets of 5 (5 x 2 x 25 = 250), then I have 50 for the power supplies which will be the ones that don't fit into sets. Does this sound reasonable?
I have matched FETs before, but never on this scale. Not sure how much variation I should expect.
I have 300 IRF610s on the way (25 kits worth). My plan is to match sets of 5 (5 x 2 x 25 = 250), then I have 50 for the power supplies which will be the ones that don't fit into sets. Does this sound reasonable?
I have matched FETs before, but never on this scale. Not sure how much variation I should expect.
I asked about the transformer spec strictly as a design question. It appears that there were a lot of discussion on this transformer issue and I'm not sure whether it has been resolved or not. I wanted to make sure that it'll work for this pre-amp, not too concerned about shipping charges.
Great to hear that you have ordered spares.
Great to hear that you have ordered spares.
Well, I didn't really. The plan with the site is to keep the products in stock for the long term, so people can order them at any time and don't "miss the group buy."
The output of the circuit sits at approx. +35V, thus the need for the output coupling caps, so lowering the negative rail won't have any effect on the output voltage swing. The lowest the ouput voltage before the output caps will ever swing is about +10V relative to ground.
The negative rail can be operated at as little as -7.5V, but at those levels the CCS behaviour is less than optimal. -10V would be the minimum I would recommend for decent performance, and there are further sonic improvements up to around -15V. Beyond that the performance difference is negligible, at least to my ears.
Exactly correct!
Cheers, Terry
metalman said:
Hi Terry,
This means the the PE trans should work fine? Any other concerns with that trans?
Thanks, Terry
Rail voltages
Lookig at this, the 1500 ohm resistors are eating up all the voltage. The drop is right at 60 volts.
In most applications this is overkill.. The swing is close to power amp level right now.
George
Lookig at this, the 1500 ohm resistors are eating up all the voltage. The drop is right at 60 volts.
In most applications this is overkill.. The swing is close to power amp level right now.
George
I also inquired about group purchase.
If we bought all they have(27) they would be 25.34 a piece, but one person would buy and reship, hardly worth it to save $4.
Glad to hear they should work!
If we bought all they have(27) they would be 25.34 a piece, but one person would buy and reship, hardly worth it to save $4.
Glad to hear they should work!