Output Cap in Rotel CD Player - diyAudio
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Old 19th April 2006, 07:55 PM   #1
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Default Output Cap in Rotel CD Player

Greetings all,

I am posing this question here as I have posted this in the tweaks asylum. I have received numerous suggestions on OTHER mods and things to do, but no one has been able to answer my question directly. They are wonderful suggestions - some of which I also plan to try later, but this is my question for now.

Short history...it's an old player that I play around with periodically - not my primary source. Once in awhile, I get the 'mod' bug. I have done the usual opamp swaps with sockets, bypassing of electrolytics, vishays here and there, hexfreds, etc.

The output section of the CD (970bx) looks like this after the opamp, and after the feedback loop:


------------->100uf---------------->100ohm-------------->Output
....................................|
....................................|
....................................v
...................................22kohm
....................................|
....................................|
....................................v
...................................gnd

The periods (.) are there just so everything lines up in the diagram. The 100uf is electrolytic (stock Black Gate) the 100ohm is Vishay (I swapped for original plain metal film).

I want to swap the BG for a film cap; however, 100uf film caps are BIG and EXPENSIVE for a piddly experiment like this (yes, I already have the BG bypassed as is).

I just want to see what a straight off film cap would sound like in place of the BG. I am thinking of possibly swapping the BG for a film of lets say 10uf (10x less cap.) (smaller and cheaper).

Can I just do the swap, or should I up the 22k resistor to 220k (10x more resistance)? What would be the ramifications of upping it versus not upping it?

Any input appreciated.

Wayne
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Old 20th April 2006, 03:02 AM   #2
lndm is offline lndm  Australia
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A 3.3uF cap will go into 22k at around 2Hz. Even less than 1uF would be sucessful based on that.

Increasing the 22k value will probably be ok but before trying that, consider this.

If the input impedance of your amp is 10k for example, the law of diminishing returns is going to make changing the 22k resistor relatively unnecessary. The absolute minimum desirable cap value would be 1uF and anything up to 10uF would be excellent.

I think it is easier to get the highest quality caps in less than 10uF sizes and so would consider this before going higher.
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Old 20th April 2006, 04:55 AM   #3
thejohn is offline thejohn  South Africa
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Remember the cap 'sees' 22k in parallel with the input impedence of the next piece of equipment + 100 so if you assume 10k as above then a 3.3u cap and 6.9k give you about 7hz... you need closer to a 10u cap for 2hz
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Old 20th April 2006, 06:19 AM   #4
Vidalgo is offline Vidalgo  Israel
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Remember the bigger physically (plus long leads) cap acts like bigger pick-up coil...
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Old 20th April 2006, 06:24 AM   #5
wenye is offline wenye  China
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Why you want to replace the BG cap?
Black Gate is a good capicitor. I don't think you have any necessary to replace it with a film cap.
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Old 20th April 2006, 08:40 AM   #6
Vidalgo is offline Vidalgo  Israel
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Quote:
Originally posted by wenye
Why you want to replace the BG cap?
Black Gate is a good capicitor. I don't think you have any necessary to replace it with a film cap.
I agree. BG is the finest option when you're need big capacitance value.
And p.c.b. is not designed to fit gigantic film cap - you need to glue it on air. Long leads isn't good thing.

btw, in my RCD-951 I have bad luck when tried to replace output capacitor from stock BG-FK to something else with same value. Tried non-polar Nichicon Muse and non-polar Cerafine - both are bad.
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Old 20th April 2006, 01:47 PM   #7
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Thank you all for your input! For clarification, the preamp is an Audio Research LS3 which has a 50k input through the normal jacks and 100k through the "Direct" jack (bypasses Selector, Stereo/Mono switch, etc.).

Yes, BGs are great caps when you need high capacitance and it does sound quite good as is with mods. But I think this is where some of the cost issues were controlled.

As expensive as BGs are for electrolytics, they are still cheaper than even smaller high quality film caps (not to say that cost necessarily relates to quality - especially sound quality). It's an excellent trade off between sonics and cost.

With that said, I just want to see EXACTLY what difference, for better or worse, it would make - at least in the context of my system and to my ears.

I think I will try a 10uf alone first to see how it is, if that's no good I will up the resistor, and if that's no good, plug the BGs back again.

Thank you, again, for actually answering my question rather than going into other mods. I do appreciate the other forums suggestions, but was really looking for an answer to my quesion.
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Old 20th April 2006, 03:28 PM   #8
Vidalgo is offline Vidalgo  Israel
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wtyamamoto,
take a look on components around RCA socket. Sometimes there is a small value (1-10nF) film or ceramic capacitor from output to ground. It's role to prevent RF pickup from outside of the device case (from mobile phones, etc.). But this cap also forms lowpass for output signal. You can try to lower its value, or remove at all. Remove at one channel and listen. No cost tweak.
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Old 20th April 2006, 06:33 PM   #9
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Quote:
Originally posted by wtyamamoto
[snip]With that said, I just want to see EXACTLY what difference, for better or worse, it would make - at least in the context of my system and to my ears.

I think I will try a 10uf alone first to see how it is, if that's no good I will up the resistor, and if that's no good, plug the BGs back again.[snip]

... and remember, the result of lower cap value is less low frequency output. With the values recommended (10uF) you most probably wont notice it, but if you go lower in capacitance it may become audible (weaker bass). So, it's easy to experiment with YOUR system in YOUR room, and go to lower / cheaper capacitance if the bass holds up fine.
This is quite a different effect (and quite predictable) from other possible audible results of going to a different type of cap.

Jan Didden
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Old 20th April 2006, 11:26 PM   #10
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Rotel uses STANDARD type Black Gate coupling capacitors for output audio coupling (and for power supplies decoupling). MUCH better (tried many times) capacitor would be 4.7mF / 50 V Black Gate C type (for coupling), or AURICap 4.7v 200V. The later is quite big but would suit “Rotel sound” better.

Extreme_Boky
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