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Old 14th April 2012, 09:18 PM   #131
Goetz is offline Goetz  Germany
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Quote:
Originally Posted by UV101 View Post
I couldn't get the Bursons to run in the power stage (the amp stayed in protect mode) but the pair in the pre were a revelation!!

Apparently the current supply in this point of power amp is too low.
You need at least 19mA but 50mA is ideal for Burson. (I guess that 12-25V is available in this area?). Pls look for the resistor which is supplying the current and reduce it accordingly to reach ~ 50mA.
Then you will reach the next level of excellence...

I used sucessfully Burson DualOpAmp in Phonostage of modded Luxman taking care of right "environment" re. V and A.
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Old 13th June 2012, 11:20 AM   #132
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Originally Posted by UV101 View Post
Hoping you're still watching this thread dtses!

Did you ever check your biasing?

....

This table is based on 0.15r
1.5mV = 10mA
4 mV = 26.7mA
8 mV = 53.3mA
10mV = 66.7mA
15mV = 100mA

I've set mine to approx 10mV so 66.7mA and wil run it for a few hours to see how it behaves but the initial indication is that it is very dynamic (not that it wasnt before) and eager. We'll see where it goes.
Cheers Ian,

Finally I've got 4 matchig pairs of new old stock genuine Toschiba transistors
Replaced them yesterday along with 8 x 0.15R power resistors - IRC U.S. 1% placed in parallel. I have also swapped both 1k trimmers to new blue high precision ones to adjust the bias carefully.

So now will play with the values.
Just to confirm, 50mA current is the sum of all 4 resistors per channel, right? Perhaps some additional heatsink would be good if I would like to go further?

Any new mods, btw?
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Old 13th June 2012, 10:37 PM   #133
UV101 is offline UV101  England
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I think I set the volt drop across the resister to give about 50mA but I should really double check. According to my posts I started at 1.5mV per resistor and settled on 7.5mV. That's about 50mA. You could try a bit more if you have lots of ventilation!

Only mods I've done at the moment is to temporarily fit lme49720ha in the preamp instead of the bursons. The reason was that I think the bursons need better regulation to sound best. I've started to build an off board PSU for the preamp and I'll run that into a couple of my regs which will hopefully sort the pre section out once and for all.
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Old 15th June 2012, 06:58 AM   #134
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Yesterday I set the bias of about 4.9-5 mV on every power resistor and then had a quick test. Bloody hell, that was something, I was shocked from the very first seconds of Metallica's My world - so much air, the scene was splendid and every instrument was separated as I haven't heard before, especially the drums.

The bad point is that Restek is very hot now even on the low volume, haven't tested it on 11 o'clock as I usually watch movies. I think it would be necessary to add some additional heatsinks on top of existing ones...
Perhaps some aluminium 15x50-70x250mm would help a bit?!

Regarding new genuine Toschiva transistors, I still have some discrepancy between 3 out of 4 pairs, like 4.8mV and 5.4mv or even 4.3 and 5.8mv, though anyway I managed to set every pair to 10mV +-0.1.

Not sure if these changes are the result of biasing or new genuine transistors' affect or adjusting DC offset by replacing 1k pot to multiturn one, nevertheless I'm very happy with the changes will post few pics later.
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Old 25th June 2012, 08:39 AM   #135
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as promised, I know everybody likes pics

So it's about 5-5.5mV on every transistor now and I get 50 c on the roght heatsink and 46 c on the left with a digital thermometer. A bit confused regarding the difference as all 4 pairs set to equal current or 10.6mV voltage. The amp gets hot after some half of an hour and although thermometer shows only 50 c it's quite hot to hold the heatsink with an arm for more than few seconds. That's with the closed lid. I will add 15-20mm of similar heatsink on top later, hoping to have few more turns of the 1k pot to go for 6-6.6mV.
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Old 6th October 2012, 10:16 AM   #136
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cheers!

could anyone advice me on connecting step DALE attenuator to the amp please?! This should be a considerable improvement to my mind!

What I want is to completely remove PRE section (10k Alps + opamp buffer + 2 opamps). The thing is that I'm a bit confused with the signal that goes to 22uf cap (ELBIP2) and 2 grounds - GNDA and GND. There is a 2 stranded shielded cable that goes from PRE to the POWER section of each channel. The screen is GND as far a I see from the scheme. Both GNDAs are connected to each other, I assume this should be the ground of my DALE attenuator, shouldn't it?
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Old 6th October 2012, 10:15 PM   #137
UV101 is offline UV101  England
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I'd use the single ended input into the power section as you suggest by picking up on the cable that links after the final opamp across into the power section. Are you going to retain the input switching or set it up purely as a power amp? You could take the input to the attenuators from after the switching section before the 1st Opamp??? ;-)

Also I think I know why the burson won't work in the power stage. The Opamp is used to create a balanced signal from the single ended input. It looks like the Opamp works in a strange way with the output fixed meaning the rails will fluctuate instead of the output. Not seen this type of configuration previously. Anyone know what this type of configuration is called??
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Old 8th October 2012, 08:26 AM   #138
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hi Ian,

thanks!
The initial idea is to retain switching, change active pre section to LDR thus making the amp an ultimate killer

The other goals are:
-compare pre section with passive DALE attenuator
-compare power section with MyRev FE that I'm about to finish

Quote:
Originally Posted by UV101 View Post
You could take the input to the attenuators from after the switching section before the 1st Opamp??? ;-)
I believe that's what I need, though not sure how to do that. 1st opamp is a buffer and has 2 inputs - inverting and non inverting, which one do I need to connect to my DALE attenuator?!
Strange that they did not separate pre and power sections like in the Challenger, but it shouldn't be difficult I believe, just asking as don't want to cook anything again
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Old 8th October 2012, 08:46 AM   #139
UV101 is offline UV101  England
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You should be able to trace the input from the relay switching section through to the Opamp. The service manual does not show the full preamp circuit!!! Assuming the Opamp shown in the diagram is the 1st stage after the switching, you need to lift the feeds from before R19 & 20. I would definitely trace the circuit forwards from the output of one of the relays towards the 1st Opamp to double check. Input should end up at the +(non inverting) input. Pins 3 & 5. Remember if you go passive pre, you'll loose the filters that's exist in the active pre section (could be a good or a bad thing!!!)

I'm definitely interested to see how you get on
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