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Old 2nd November 2006, 01:37 AM   #1
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Default Cyrus Two low input impedance?

Does anyone know if the Cyrus Two amps have especially low input impedance?

The Cyrus website quote 14KOhm. http://www.cyrusaudio.com/product.as...D=71&text=spec

I have just purchased a (pre-assembled) tube buffer stage which goes before this amp and the maufacturer is telling me that the quoted impedance is only a nominal value and that the amplifiers low impedance is causing the distortion problems with the preceeding tube buffer!!

Does anyone know how difficult it is to drive this amp?
It's preamp section is passive, if that helps.
I'm just hoping that someone might know since theses amps are quite popular.
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Old 2nd November 2006, 01:53 AM   #2
anatech is offline anatech  Canada
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Hi Sonusthree,
The input bias resistor is 75K with a 4K7 resistor in series. This would be in parallel with the wiper of the volume control (10 K). Therefore, your input resistance is a little less than 10 K. Since there is a blocking capacitor, why not install a 100 K pot instead? The taper will not be exactly correct. This should help reduce the loading on the tube buffer.

I am assuming you just bought the buffer to lose the edgy sound some amps can get? What you ought to do first is recap your Cyrus. Replace all ceramics with mica or foil caps. Replace low value electrolytics with foil types and all other electrolytics with new. This is very important for the phono power supply. Almost every unit I see has bad caps near the voltage regulators (6 I think). This will make a large improvement in sound quality.

-Chris
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Old 5th November 2006, 11:34 AM   #3
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Thanks Chris.
I've already replaced the standard pot with an Alps Blue velvet. It wasn't too easy getting it to fit so I'd rather not change it again if I can help it. (I'll have to check the value later today but I'm sure it's no more than 20K?)

As usuall your advice about recapping is spot on. You read my mind about the edgy sound. I couldn't see the wood for the trees!! I've begun ordering the electrolytics already and wil post my results here.

I use my Cyrus 2 with the PSX and so do not need the internal supply to work at all untill I can find a bargain record deck. Still looking though. Ill recap that later on.

I recently contacted Cyrus about their renewal service for the PSX. I thought that it might be best to get the four monster 15000uF slit foil caps directly from them since they are quite hard to get hold of in the UK. They said:

"We do offer a service for the Cyrus 2 but the PSX is very stable and there would be no enhancement by offering a restorative service. With regards to the capacitors, we do not have these in stock as we do not change these as a rule."

16 year old 'lytics make me nervous but what do I know?

The Tube buffer is the only reason that I am asking about input impedance. The maker tells me that the input impedance is probably too low on my amp and that is why the Tubalizer is distorting badly. Frankly, I don't believe him since it distorts with every piece of equipment I have in the house. The only time it doesn't distort is when placed between the pre and power sections of a NAD integrated and this is because the input signal is greatly reduced by the preamp. I have tested on my soundcard and basically the negative cycles are being clipped but I guess this is an issue for the Tube forum.

Thanks again for your help Chris.
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Old 5th November 2006, 05:21 PM   #4
anatech is offline anatech  Canada
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Hi Sonusthree,
Change the 6 caps for the regulator stage now. Add some heatsink grease to the regulators now also. They are currently oscillating I bet, and running too hot. Given the cost you could always install new regulators now also. This cuts down on the number of times your unit has to come apart.

Check for ceramic caps at C69 and C70. Replace with mica. Do all teh other electrolytics as well as the 470uF 6 V feedback to ground caps. A bypass with a poly type would helps here. There are already a large number of polystyrene caps (great news).

You can replace the main filters later on. The PSX would be working within spec, but maybe conside bypassing them locally with film types (just the supply in the amp). On second thought, bypassing the main caps in the PSX would reduce the high frequency noise coming across the cable. Might be worthwhile.

-Chris
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Old 5th November 2006, 06:33 PM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
I don't think the PSX disables the internal PSU.
The low current stages still get their power from the internal and the power output stage gets it's power from the PSX.

part of the improvement comes from the bigger grunt of the PSX, but I believe that a bigger improvement comes from the SEPARTION of the supply to the low level stages and the high current stage. No modulation effects creeping into the voltage amplifier.

If all the preceding is true then replacing aged components in the internal PSU should also reap dividends.
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Old 5th November 2006, 09:16 PM   #6
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As far as I understand it the PSX replaces the internal Cyrus 2 power supply except for the phono stage and the front power LED.

I provide power only to the PSX in my setup since I don't currently own a vinyl source (Yet )

Once I get the rest of my system balanced I would certainly like to hear the Phono stage which is, apparently, fantastic. Presumably partly due to the power supply seperation mentioned above.
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Old 5th November 2006, 09:20 PM   #7
anatech is offline anatech  Canada
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Hi Andrew,
The PSX replaces the supply for the entire amplifier including the voltage amp stage. The internal supply runs the power LED (yahoo!) and the phono amplifier regulators. That's it. However, the internal regulators tend to oscillate when the supply caps go bad and this could interfere with the operation of the power amp. Since these supply components are not expensive, it's best to simply replace the lot and be done with it.

Some amplifiers do regulate the voltage amp stages independently (like my Marantz 300DC). This really does have a very positive influence on the sound quality.

-Chris

Edit: Hi Sonusthree, I see we posted close to the same time.
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Old 5th November 2006, 09:31 PM   #8
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Quote:
Originally posted by anatech
Edit: Hi Sonusthree, I see we posted close to the same time.
I guess I must be learning something!
The funny thing is that ever since I've been a member here it's been both yourself and AndrewT that have helped me to learn the most about my amp.
Seriously guys, thanks.
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Old 5th November 2006, 09:36 PM   #9
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Hi Sonusthree,
The phono stage was very well designed for it's era. Very much an op amp preamp. I just figured the less you take your Cyrus apart, the better off you will be in the long run.

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Old 12th November 2006, 12:54 AM   #10
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Default A portion of Distortion.

Hi.
So far I have pretty much completed replacing the electrolytics, I have added heatsink grease to both regulators. I have used Cerafines for the polar and Black Gate N for the Non polar. I have managed to fit (just about as a temporary measure) two huge 2.2uF MKP's at the input of the power amp section. I have replaced the two 470uF caps at the regulators with 680uF.

I have not changed the large slit foil caps on the PSX or amplifier's internal supply.

I'm having a little trouble finding suitably sized 2.2uF Film caps to replace the original electrolytics but I believe these are for the currently unused phono stage. New regulators are on their way.

The sound at the moment is very disappointing. I'm hearing some distortion in both channels and the soundstage is greatly diminished. I would describe the problem as a very strong "Grain". It appears mostly when singers put some vigour into their performance.

I'm just starting to try and come to a diagnosis. I have disabled the PSX and a using the internal supply. I've tried different sources at differing levels with no difference. It seems to me that it is frequency related so I will run some test tones tomorrow.

I was expecting some time for the caps to settle. I've only run it for approximately 18 hours so far since the recap but I don't think this is the cause.

It's driving me crazy. Partly because I've tried so hard to improve the sound and have made it worse somehow and perhaps more importantly because I feel completely out of my depth if there is another more serious problem.

It seems to me that it is frequency and not level (input or output) related so I will run some test tones tomorrow.
The problem is in both channels.

My suspicions so far are C43 (470uF 16V) in the Feedback loop. I replaced it with the same value Black Gate N. The original looked quite strange when I removed it. Kind of potted at the bottom. I'll post some pictures soon. Perhaps the low ESR of the new caps has altered something? The originals were mostly Elnas though. There was already some very slight distortion before the recap but definitely not as obvious as it is now.

Please forgive my inane ramblings here especially since I have not yet tried absolutely everything to remedy the problem. I just needed to get this off my chest. I have dreamed of owning exactly this setup since 1991 and it means the world to me.

Thanks for reading this far.
Martin.
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