using heresy with nad/foreplay question

[sberger]

Hi,

To start, this is my first post here, but lurking around incented me to pick up my fist pair of vintage (79) heresy's today. presently, i'm using them with my nad c370 amp while my primary amp, a sim audio i-5 is in the shop. in the past, to sweeten the nad, i've inserted a bottlehead foreplay tube preamp in the chain by plugging into the nad's "main in" jacks, which allows you to do this. but i've never used speakers that are as efficient as the heresy. the nad has 130 watts. the nad on it's own runs and sounds fine, but when i try to use the foreplay, i experience a lot of popping, static, etc, and the nad then shuts down. this didn't happen when i used the nad with other speakers, like totem model 1 signatures, which are 88 db sensitivity. i'm wondering if the foreplay, which is an active pre and has about 20 watts output or so, combined with the foreplay, is just too much for the heresy to handle.

i've posted a smiliar post in the bottlehead forum, but i thought some folks here might have comments/opinions to offer.

it's good to be here, and thanks in advance for your help!

[codhead]

[sberger]

the thing is, the heresy's work fine without the preamp plugged into the nad. in other words, using the nad's pre(it's an integrated) section. so i'm pretty sure that it's not the speakers, per se. my biggest concern if there was just too much power going into the speakers.

i have tried the nad/foreplay combo with other speakers before without any trouble, but for now, the heresy's are the only speakers i have.

[codhead]

[sberger]

just replaced the tubes and problem still exists. is there any chance that the combined power of the amp and pre is somehow causing the speakers to overload, thus shutting down the amp? seems weird, but i can't figure out what else it would be.

[codhead]

[sberger]

well, found the problem! believe it or not, it was the order of power on that i was using. i was powering the amp first, then the foreplay. wrong! foreplay needs to go first, although for the life of me, i thought i used to do it differently with my other amp. but a little research (my foreplay manual, and their forum) and there was the answer. things work and sound great now.

thanks for your replies and help.

[spaaaz]

yep the pre always goes on first and off last,that way you avoid any tube rush to the amp when it is turned on and also can do the same thing if you power off the pre before the amp,glad everything is working for ya.enjoy

[Erik Mandaville]

Hi, Codhead:

The Foreplay is a decent preamp, and, depending on the input impedance of the NAD, which is probably lower than most vacuum tube designs, might provide a useful impedance matching buffer between your CD player and the amp. However, you might want to experiment with the position of the volume control on the NAD relative to those of the Foreplay -- which could have some effect on signal-to-noise ratio of the arrangement. The Foreplay is a direct-coupled cathode follower, with an output impedance of a little over 1,000 ohms, and should work reasonably well, as those I have built have, with a variety of solid state and vacuum tube amps. What you're doing is sort of like running the line level signal from the CDP through a low impedance vacuum tube filter, and then passing that onto the preamp of the NAD -- which might lend some tube flavor to the mix. It's possible that, in a subjective sense, this setup might add a little extra complexity to your system.

On the power up sequence, and this is something I mentioned on Doc B's site years and years ago, it can be very helpful to install a low value, high voltage snubber capacitor across the switch contacts. This is an idea I got from a Dynaco preamp schematic, and its a little trick they used on equipment for a long time. AVA (Audio by Van Alstine) does the same thing, and it's possible to see this little capacitor in pictures of their equipment. The capacitor simply absorbs the pop that would otherwise be sent downstream to the amplifier/s and speakers, and works great if you forget (I sometimes do)to turn the preamplifier on first.

Something in the range of a .01uf (microfarad) capacitor with a voltage rating of 1,000 volts would do the trick very well. I can't remember if you said you built the Foreplay yourself, but if you did not and/or you do not have experience working with electronics, please don't attempt this work. It's very simple, but there is of course the danger of high voltages (EVEN WITH THE POWER CORD UNPLUGGED)hiding inside the filter capacitors. Get help from somone with experience. Or, just remember to turn your preamp on first!

Have fun,

Erik

edit: I just remembered that I DID get the snubber cap idea from a dynaco schematic, but had forgotten that the schematic was in a book by Bruce Rozenblit, who thus must be credited as my source for the idea.