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 Post subject: Cheapest tweek ever
PostPosted: 29 May 2011, 18:34 
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Joined: 26 Mar 2011, 14:44
Posts: 18
Hi
I'm usually over on the valve/tube board but I play with SS amps too. I wanted to share a cheap tweek I did just recently. I own a Rotel RA 935 BX integrated that I have tweeked somewhat already. It was not bad at all but not quite right. Latest mod was to replace the bridge rectifier with four MUR420 ultra fast recovery diodes bought from CPC for about 30p a pop. Result? amazing. Gone is the slight graininess that had bugged me since I first owned the amp. Treble cleaness and detail have improved amazingly mids are sweeter even bass is so much better more organic The amp is now sounding almost tube like. This improvement for just £1.20 because I bought the diodes with another item, a new soldering iron. If anyone is interested in the other tweeks I am happy to share.

Regards
Todd


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 Post subject: Re: Cheapest tweek ever
PostPosted: 31 May 2011, 16:10 
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Joined: 14 Feb 2010, 13:13
Posts: 665
Location: Lisbon, Portugal
Hi Todd!

That´s quite an impressive change for a bridge rectifier change! I´d never remember to do such a thing. Thanks for sharing with us!

Cheers,
Miguel


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 Post subject: Re: Cheapest tweek ever
PostPosted: 31 May 2011, 18:04 
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Joined: 26 Mar 2011, 14:44
Posts: 18
Hi
Thanks for your feedback. I wasn't sure if I had posted this in the correct board.
I was pretty stunned to say the least. The amp has been tweeked a bit though and I suspect the improved rectifier has allowed my mods to show their full effect. Its strange that Rotel went to the trouble of fitting expensive Blackgates in critical places in the RA935BX but didn't use a low noise rectifier.
Rectifier noise is insidious stuff extending right up into the RF spectrum and it gets all over the place by conduction induction and radiation so we don't want it, in my valve stuff I always use thermionic rectifiers but even these need a bit of snubbing. get some of these diodes and fit em. CPC sell them for 20p +vat each. I know we can find more exotic devices but they are pricey. I like 20p + vat.....

Cheers
Todd


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 Post subject: Re: Cheapest tweek ever
PostPosted: 02 Jun 2011, 12:12 
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Joined: 28 May 2008, 21:53
Posts: 4580
Location: Winnipeg, CANADA
Todd-uk wrote:
I wanted to share a cheap tweek I did just recently. I own a Rotel RA 935 BX integrated that I have tweeked somewhat already. It was not bad at all but not quite right. Latest mod was to replace the bridge rectifier with four MUR420 ultra fast recovery diodes bought from CPC for about 30p a pop. Result? amazing. Gone is the slight graininess that had bugged me since I first owned the amp. Treble cleaness and detail have improved amazingly mids are sweeter even bass is so much better more organic The amp is now sounding almost tube like. This improvement for just £1.20 because I bought the diodes with another item, a new soldering iron. If anyone is interested in the other tweeks I am happy to share.

Hi Todd, I also like to replace bridge rectifier with individual ultrafast switching diodes. Even the inexpensive ones are typically an improvement over a bridge rectifier. In addition I also like to add snubber diodes across the diodes ~0.022uF (pick an appropriate voltage rating).

Feel free to share other tweeks you may have.

Cheers

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 Post subject: Re: Cheapest tweek ever
PostPosted: 02 Jun 2011, 15:31 
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Joined: 26 Mar 2011, 14:44
Posts: 18
Hi
Yes the diodes are snubbed by caps about 10nf I think. Re other tweaks. Local bypass caps were el cheapo ceramics, these swapped out with some of those flat orange coloured polyester caps by Philips. This was first mod and had a significant effect on the sound. Ceramic caps have no place in audio IMHO. Main PSU caps Rotel customs, were changed for Nichicon KG Gold Tune and things really started to smooth out. I bypassed these caps with mylar film caps and a pair of Blackegates 33ufs I had in my parts bin for a small inprovement again. Another mod, and some may think this is snake oil. I screened the final transistors with a copper strip this connected to the star point. This was where things started to sound really good and then last Saturday the ultra fast recovery diodes were implemented for a final refinement. Oh and I set the finals idle current at twice the factory setting for a tad more class A.
The transistor screening thing is a Ken Ishiwata (Marantz) technique I believe. I tried it years ago in another budget Rotal amp and it really does improve things. The theory is that the output transistors radiate unwanted and chaotic feedback to the small signal stages. I attach a photo. The Philips caps had been swapped out again for comparison now back in for good.
Attachment:
RA935BX copper.jpg

Why a Rotel RA935BXamp? I like the circuit layout it being a very carfefully designed star point system its got blackgates in critical places and I got the amp for 40 quid off Ebay, nuff said.
Ok for now


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 Post subject: Re: Cheapest tweek ever
PostPosted: 02 Jun 2011, 17:56 
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Joined: 14 Oct 2008, 17:35
Posts: 901
Location: Dartmouth, Nova Scotia
Great mods Todd :up:

I think I have seen one of these stereos at the place where I work too. The diodes and the copper plate holding down the transistors are interesting mods. :smoking:

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 Post subject: Re: Cheapest tweek ever
PostPosted: 03 Jun 2011, 04:40 
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Joined: 26 Mar 2011, 14:44
Posts: 18
Hi
My other amp is homebrewed SE tube but I do like to mess about with SS stuff. I bought the Rotel as a standby amp and the tweek-itis just took over. I now prefer the Rotel over my SE amp for its well defined yet organic bass and its open treble. Heresy?

The copper plate is a screen that just happens to use the original transistor clamping screws and yes it does a good job.

Todd


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 Post subject: Re: Cheapest tweek ever
PostPosted: 24 Feb 2012, 00:15 
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Joined: 24 Feb 2012, 00:07
Posts: 1
Location: New Zealand
Hi there Todd - I have a pair of rotel rb850s and am keen to learn more on this rectifier mod? Are you able to share with me?! cheers from NZ! simon


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 Post subject: Re: Cheapest tweek ever
PostPosted: 10 Aug 2012, 06:42 
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Joined: 09 Aug 2012, 10:14
Posts: 1
Hi Todd. I guess I'm very late for my question but , being a beginner , I'm wondering if you could kindly
send me , or add to the topic , some close pictures of the other points where you tweaked the board.
Cheers. Val


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 Post subject: Re: Cheapest tweek ever
PostPosted: 02 Oct 2012, 06:41 
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Joined: 02 Oct 2012, 06:39
Posts: 1
A little voltage amplifier stage can be added in front of the PP OPS. This also will provide some extra current to drive the OPS (output stage) FETs or Transistors.

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