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Old 29th January 2005, 02:13 PM   #71
fab is offline fab  Canada
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Dick,

This is not hijacking to discuss mods on a thread titled "Hafler" "mods"!

I can not see problem with running the driver board 5vdc more than the stock XL-280 supply since all components on the board are rated with a big voltage margin. However, I saw in the manual that the XL-280 has the big 7800uf caps rated at 75vdc. The voltage margin between 75V and 70V is not that much if you consider that the main 120Vac voltage has variation and also during power up you can have short spikes. I do not know the real effect on long term operation. Check on the caps if they are rated 75vdc (as nominal) but max voltage 90 Vdc (transient).

I can not see a problem running the output stage including the MOSFET and their associated driver transistors Q14 and Q15 with a lower voltage. As you have indicated you will loose a lot of max output power (maybe only about 85 W rms clean power compared to the rated 140W rms with the original transfo).

Fab
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Old 29th January 2005, 02:58 PM   #72
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fab,

The stock XL-280 has circuit cards designed to operate at 75VDC. The stock XL-280 transformer has two sets of secondaries, one to provide around75VDC, the other to provide 65 VDC for the output stage. Therefore, there is no concern about using my XL280 circuit cards with 70 VDC.

Also, the 75WVDC rating of the PS caps is not a problem as the output stage of the stock XL-280 runs at the same voltage as the DH-200 and DH-220.

As you know, the XL-280 has an input circuit that runs at 5-10 VDC higher than the output stage. This compensates for the 5 VDC gate voltage to get the MOSFETs to conduct. This design gets the max power out of the components, chassis and heat sink size, possible.

Thanks for the data about power reduction using 50 VDC instead of 65 VDC for the output MOSFETs. I can live with 80 or so watts as my speakers are efficient and I am not a loudness freak. I hope that the dual mono amplification will provide an enhanced sound stage and clarity that overcomes any problems with an occasional lack of power.

Well, whatta' hobby! Right?
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Old 29th January 2005, 04:00 PM   #73
fab is offline fab  Canada
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Default dual mono amp

Dick,

it is surely a good idea to have individual power supply for each channel since it mainly prevents modulation of the power supply (of one channel) by the other channel musical signal. Do you intent to increase the caps bank value as well?

I have not done this mod but only thought about it. Keep us informed on the "before and after mod" difference (with same musical material and source of caourse).

I would also consider regulating the driver voltage since you have a lot of margin (75V to 50V) to be able to still keep the max power unchanged. Even a regulated 65V would be fine. By the way the SK134 and SJ49 are lateral mosfets and they are polarised to about 0.75Vdc (check R36 voltage that is probably to about 1.5 Vdc for bias of about 280 ma) and not 5Vdc as V-mosfet type.

fab
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Old 29th January 2005, 04:04 PM   #74
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This looks like a good time to ask something that I've been wondering. I own a couple of 280-XL trannies. Is the amperage on the 70V secondary the same as the 65V? Could I use the 70V winding instead of the 65V if I wanted the extra voltage?

Thanks, Terry
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Old 29th January 2005, 05:38 PM   #75
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The HV winding on an XL280 transformer is only rated to run the front end cards, 100mA or so.

Remember: 134/49 devices are only rated at 140V MAXIMUM and 135/50 devices should be used with the Dynaco 300 70V windings.

Sound Valves has some XL280 transformers left if you really want to go dual mono with your pair of XL280 chassis.

"Are these the CL30 you and others recommend. Mine don't have the "L" in their description but I assume they are the same thing. Am I correct?"

Sounds like a similar part from a different vendor, I would probably use it.

I would save the pair of Dynaco 300 transformers for a different project.
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Old 29th January 2005, 05:58 PM   #76
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I realize my post about using the Dynaco transformer contained too many parts for most to read carefully.

I do not intend to run the output MOSFETs at more than 50VDC.

The Dynaco trans. has two sets of secondaries, one provides 70VDC the other 50VDC. The circuit cards will run at 70VDC and the output MOSFETs at 50VDC. That is what I intend to do. That pair of old Dynaco transformers really bug me and I want to find a reasonable way to use them.
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Old 29th January 2005, 06:24 PM   #77
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fab,

For several years here I ran a pair of monoblocks made from a DH-200. Each had its own power supply with the heatsink on the left and the tranformer on the right to balance the weight. In between was 80,000 mfd of PS capacitance and a soft start surge resistor/time delay relay

The monoblocks threw a big wide soundstage though in retrospect it was a little diffuse. Then along came a MusicalConcepts amp with the PA-3B circuit cards and 50K uF in the PS. This amp's soundstage was almost as large as that of the DH-200 monoblocks but it was more detailed, more subtley nuanced, not diffuse, and with better placement of sound images. It became my reference amp.

Now, into the two empty chasses I could mount the XL-280 cards with the Dynaco transformer and the 80K PS capacitance and maybe even regulate the front end voltage from 70 down to 60 VDC to make sure it gets not only regulated but decoupled from the 50vdc windings that run the outputs.

The bottom line, however, how will I like the XL-280 sound compared to that of the MC amp? Mybe I should just split the MC amp and make it dual mono, run everything at 70VDC and use the matched triplet sets of 2SK176/2SJ56 MOSFETs recently acquired.

But, how does the "sound" of the XL-280 compare to that of a MC amp with the PA-3B circuit cards?

Any comments about the XL-280 "sound?"
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Old 29th January 2005, 06:56 PM   #78
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Default XL-280 - MC

Quote:
Originally posted by Dick West
fab,

For several years here I ran a pair of monoblocks made from a DH-200. Each had its own power supply with the heatsink on the left and the tranformer on the right to balance the weight. In between was 80,000 mfd of PS capacitance and a soft start surge resistor/time delay relay

The monoblocks threw a big wide soundstage though in retrospect it was a little diffuse. Then along came a MusicalConcepts amp with the PA-3B circuit cards and 50K uF in the PS. This amp's soundstage was almost as large as that of the DH-200 monoblocks but it was more detailed, more subtley nuanced, not diffuse, and with better placement of sound images. It became my reference amp.

Now, into the two empty chasses I could mount the XL-280 cards with the Dynaco transformer and the 80K PS capacitance and maybe even regulate the front end voltage from 70 down to 60 VDC to make sure it gets not only regulated but decoupled from the 50vdc windings that run the outputs.

The bottom line, however, how will I like the XL-280 sound compared to that of the MC amp? Mybe I should just split the MC amp and make it dual mono, run everything at 70VDC and use the matched triplet sets of 2SK176/2SJ56 MOSFETs recently acquired.

But, how does the "sound" of the XL-280 compare to that of a MC amp with the PA-3B circuit cards?

Any comments about the XL-280 "sound?"
Dick,

unfortunately I can not really provide a valuable opinion on the sound between these options since I have not heard either of them. However, I know that the circuitry of the MC PA-3B circuit includes a cascode VAS which might (it is only a possibility not a verified fact for the MC) be partially responsible of the "more detailed, more subtley nuanced, not diffuse, and with better placement of sound images." In 2 of my DH-200 amps, this is one of the mod I have done and I can say that I have the same comments on the sound as you when I have compared with the stock DH-200. However, I have performed several mods so it might be other mods that really made the difference also.

fab
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Old 29th January 2005, 07:02 PM   #79
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fab,

As I recall from a post many months ago you toyed with the idea of making a PCB that encompasses your DH-200 mods onto a circuit card which others can use to enjoy the fruits of your labor and knowledge. Any further thoughts on this?

I praise MC circuit cards highly but the price of their kit is more than I want to pay, at least not pay frequently. How about a PCB from you?
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Old 29th January 2005, 08:24 PM   #80
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Quote:
Originally posted by Dick West
fab,

As I recall from a post many months ago you toyed with the idea of making a PCB that encompasses your DH-200 mods onto a circuit card which others can use to enjoy the fruits of your labor and knowledge. Any further thoughts on this?

I praise MC circuit cards highly but the price of their kit is more than I want to pay, at least not pay frequently. How about a PCB from you?

What Dick said!

Blessings, Terry
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