burnedfingers said:
I think it's a bad case of capacitoritis -- the line amp and tone amp utilize the same chip -- coupled by old electrolytics -- and the tone controls are just awful.
Tone controls aren't necessarily a bad thing - in this case the "tone cancel" switch will be used to engage a separate loop. The "Panorama" control is going to require some obviation. There was a circuit in this weeks EDN which deals with this exactly.
So here goes -- the line amp is replaced with a compound amplifier, the tone control loop is isolated with its own control circuitry, phono preamp replaced, power supply and regulatory circuit is replaced with super-regs, transformer and power diodes snubbed, the pan "fixed", and interconnects replaced.
I installed a super-regulator (with pre-reg), replaced the 250uF 35VDC filter caps with 1,000uF and plugged in a pair of the new National LME49710 chips -- the first distortion tests were pretty good considering that the negative channel of the super-reg is a bit noisy -- on the order of 0.003% THD which is a tenforld improvement over the stock unit.
the super-reg wiring is a little sloppy and I have to figure out why the negative supply is a tad noisy --
the super-reg wiring is a little sloppy and I have to figure out why the negative supply is a tad noisy --
An externally hosted image should be here but it was not working when we last tested it.
FastEddy said:" ... the negative channel of the super-reg is a bit noisy -- on the order of 0.003% THD ..."
What do you use to measure this ?? 'scope on a rope & software? ...
I use a Tek AA501A with an SG505 oscillator...distortion is in the low 3/1000's of 1% -- should be able to get it down to 1/1000. I also look at the distortion residuals on a spectrum analyzer.
... maybe we should make some, DIY ...
It isn't difficult to make a very, very good oscillator for one frequency -- search Super Oscillator -- it gets tricky when you make an oscillator which will sweep from a few Hz to 150kHz. to anlyzed distortion you measure the rms values of the residuals -- (you square the values, take the square root of the sum etc., etc.) this can all be done with a HP3581 wave analyzer (or the Genrad equivalent) and a programmable calculator.
i have been thinking of doing this in Visual Basic with the FFT component in Softwire -- you need a fast ADC, however, to get useful results, something much more expensive than the USB1208FS from Measurement Computing which is rated at 50ksps single channel.
i have been thinking of doing this in Visual Basic with the FFT component in Softwire -- you need a fast ADC, however, to get useful results, something much more expensive than the USB1208FS from Measurement Computing which is rated at 50ksps single channel.
I've had a couple of these things, and thought about a rebuild, but sold them. The voltage reg was the incredibly unreliable Raytheon 4195. These had a production problem in the early runs and failed frequently. Don't know if they ever solved it, as we used them where I used to work and had the same problems. Sound quality was pretty bad, though I thought my tone controls behaved more predictably. The remote turn off feature was actually very handy. I wired it to a small switch by the telephone, so I could kill the sound when the phone rang. I've heard that the IC150, non-A model, sounded a lot better, but I've never listened to one. Did like the solid chassis, though the look is a bit dated. Fun project.
The 4195 was replaced with an LM7818 and LM7918 -- or their Motorola equivalents...wired right over the location for the 4195.
I think that it was in pretty bad taste that the rectifiers are on the main preamplifier board --
and they worked to save money by having the main board and the phono board mechanical connections made via the shafts of various potentiomenters.
i think that the really bad sound was due to the opamps and inattention to the power supply.
there is plenty of room for upgrades, you don't see the Siemens polystyrene caps anymore, the attenuator is, in fact, pretty decent...
I think that it was in pretty bad taste that the rectifiers are on the main preamplifier board --
and they worked to save money by having the main board and the phono board mechanical connections made via the shafts of various potentiomenters.
i think that the really bad sound was due to the opamps and inattention to the power supply.
there is plenty of room for upgrades, you don't see the Siemens polystyrene caps anymore, the attenuator is, in fact, pretty decent...
OK, that's it -- finished -- I know THD% isn't the absolute argument, but it's now 0.0010% -- the SuperRegulator AD825 had a little hair of wire shorting out the input pins -- replaced the NP output cap with a little polypropylene number -- reduced the lenght of the wiring from the super-reg to the preamp board to about 2 inches -- i guess the total cost of the pooge was about $20.
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