Krell KAV-250cd Output stage drawn

[Zero Cool]

I picked up a Krell KAV-250cd output stage board (non MKII) and spent some time drawing out the schematic. fairly unimpressive actually. I did not drawn out the D/A and I/V stages as i was more interested if the board could be used as a line amp.

They are using an OPA2604 opamp to convert from singled ended to balanced drive to a set of bipolar output sections comprised of MPS8099 and MPS8598 transistors.

These boards are dated 09-98 so it's an old design now but it was interested to get a glimpse inside the design...


Zc

[Johnloudb]

yeah, I don't see the point in the bipolar output stage.

[Zero Cool]

its an emitter follower stage that provides current to drive long cables. Now whats interesting is the MKII version board is MUCH bigger with many many more transistors. Now whats not show in this schematic is that there are 2 more opamps that are before this stage and after the PCM1702D/A converters. I have not drawn out that part of the circuit yet. it looks to be either a I/V stage or maybe a filter. the board was also supposed to have a PMD HDCD chip but that is long gone too. Im curious if the MKII board they did all discrete instead of opamps.

[Speedskater]

Somewhere in the back issues of "The Audio Critic" David Rich comments on the somewhat unusual Krell designs.

The Audio Critic

[eradoncic]

Originally Posted by Zero Cool

Im curious if the MKII board they did all discrete instead of opamps.

I have the Mk2 version, and yes, it is a full discrete component board around PCM1704 and PMD100 HDCD digital filter.
Also it has changed transport (now it consists Daisy short loader mk2, based on Phillips CDM 12.01)
The only op amp on that board is 0P97 but it seems to have some servo role - difficult to say wthout schematics

[Johnloudb]

Originally Posted by Zero Cool

its an emitter follower stage that provides current to drive long cables. Now whats interesting is the MKII version board is MUCH bigger with many many more transistors. Now whats not show in this schematic is that there are 2 more opamps that are before this stage and after the PCM1702D/A converters. I have not drawn out that part of the circuit yet. it looks to be either a I/V stage or maybe a filter. the board was also supposed to have a PMD HDCD chip but that is long gone too. Im curious if the MKII board they did all discrete instead of opamps.

I understand that reasoning, except that good cables have only about 12pF/foot and this is a CD player output stage. Most people use pretty short cables between a CD player and a preamp. I mean just stick a 221 ohm resistor at the output of the opamp and your good to go. That op amp can deliver 35mA.

So, it just seems like a bad idea to me, but perhaps there is something I'm missing.

[Zero Cool]

I stripped the DAC chips and opamp filters off the board and turned it into a Buffer with Unbalanced input to Balanced (or unbalanced) output. I calculate 12db of gain. 500mv input gets you 2v out across the balanced output. Might make an interesting low budget line stage section. going to have to listen to it and see how it sounds.