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RE: Service manual/schematic for Quad ESL-988 + modding tips?

Posted by kentaja (M) on September 16, 2012 at 09:33:58

In Reply to: RE: Service manual/schematic for Quad ESL-988 + modding tips? posted by Zombie on September 16, 2012 at 09:03:41:

Yes the schematic states 5.25kV but they never actually polarized the panels quite that high. Closer to 5.1kV. 4.7-4.8kV will drop the efficiency by aboutt 1 - 1.5dB. Provided the secondary voltage of the transformer is 670V 82K will get the bias voltage around 4.7-4.8kV. I have seen secondary voltages as high as 700V on 98 speakers. In that case I would make the R13 value around 100K-120K.

Dropping the voltage as low as possible while still maintaining good output without clipping the amp is key. I have chosen 4.7-4.8kV as the target for amps with 50-100 watts output which is what the typical Quad owner will use. I personally use 300 watts and drop the voltage closer to 4.5kV.

Later ESL-63 schematic and the 98 schematic have an error omitting C26 but it is in the speaker. C26 is right next to resistors R1A,B and R2A,B on the PCB.

C26 is 1.5uF/50V. I typically replace with a Mundorf MKP of the appropriate value. For the resistors I have used many different types depending on budget. I personally use Caddock metal films with heat sinks but standard Dale/Vishay RS-5 work nicely as well. Resistors are 3.3ohm and should be a minimum of 5 watts.

Like anything tastes will vary on passive components and other equipment in the chain. Just about anything will sound better than the stock components.

Edits: 09/16/12 09/16/12

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