His later designs were published in Audio Amateur, as the 60, DC-100 and servo 100 series. There were also a lot of reader improvements published well into the 90's including the original POOGE series that took a look at mostly grounding. I modeled the DH-120 in Spice and found more than a few things to play with. The 200/220 is a totally different design having a symmetrical IPS and VAS. Better out of the gate. Don't know if I am going to play with mine or just sell it. I suspect newer transistors would allow faster slew rates and lower noise. That means total rework of the stability looking at the poles and zeros. The IPS does not have either CCS or CM as Mr.'s Cordell and Self would suggest. Servo is always an option, as would be adding some form or protection system. Fuses will protect neither the speaker not the amp. I know this as a fact. ( want to see the invoice for the Exicons?)
I got my bridge from Parts Connection. It is an IXYS. ON-Semi makes quite a few suitable parts but not in a potted bridge assembly. My Exicon L-MOSFETS came from Profusion.
If anyone replaced the Hitachi fets, be aware the Exicons have less capacitance, so adjustment of the gate resistor may be in order. Strange, but the Hafler manual does not list values for these. (R404) so I don't know if he still used the same value on the P and the N. To get symmetrical slew, they will be different unless a compensation cap was added to the N. Don't know. Trying to second guess someone as smart and experienced as EB is not an easy thing to do! I don't run in that league. Anyway, it seems HF oscillation was a bugaboo he chased in all these designs, so any change you will need to be sure you are as though as he was. This was not an amateur design, no cook book "just do this is out there". The more you dig into it, the more yo will find out how good he is.
I took on the 120 as a learning experience. Learning about it cost me more than just buying a much newer and better amp, like any of the Curl designed Parasounds, newer Rotels etc. Heck, An Aragon went for only 400 and an Acrus for under 200! Much easier and cheaper way to get better sound.
I am about half done:
Textbooks, 3 for about $200
Various parts orders, about $200
Variac $100
A couple months intensive work, bugging really smart people, and learning how fascinating and difficult amp design is: Priceless.
Heck, If I had just sold about 5 of my amps to start with, I could have bought a Krell. Not the point of a hobby though. What I want to know is why the old small Rotells sound so darn good with such simple designs. And of course, does a few simple tweaks like IPS emitter degeneration help their sound even more?
- Status
- Not open for further replies.