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Old 11th August 2007, 09:16 PM   #1
tkwou is offline tkwou  Canada
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Default Mission 776 preamp mods

I recently picked up a non-working Mission 776 preamp on Ebay. The problem was just with the power supply so I was able to get it working fairly quickly. It is an unusual preamp in that it runs off a 24V battery and has a built in recharging and trickle charging unit which turns on when you are not using the preamp. The other unsual thing about this preamp is that mine has a MC only phonopreamp. I was totally unaware that this existed but I must assume this particular version was introduced to match their 773MC cartridge line. I have tried emailing Mission/Cyrus but they have not responded. I have traced out the circuit and it uses fairly simple discrete circuitry using constant current sources for all the collector loads. The input is quad parallel NPN BJT CE amplifier that is direct coupled to a PNP CE amplifier. This output is then capacitor coupled to a differential pair which is direct coupled to a another CE amplifier. The output is then capacitor coupled. The RIAA circuit is through a feedback loop to the opposite side of the differential pair. The preamp sounds pretty good as is but I think that it can sound better with some component changes. This preamp came out at a time when capacitor sound was coming to the forefront. Noticably they have used different brands of coupling capacitors throughout and have bypassed these capacitors with smaller value film caps. From what I can see they are mainly polyester caps from Rifa. These polyester caps are also used to bypass the big electrolytic caps in the power supply flitering. Also the input coupling cap is a 220uF tantalum which aren't known to be sonically great. I don't know if I should just bypass this with a good polypropolene or replace it with a good electrolytic too. As for the polyester coupling caps I think polyprolene would be sonically better but I'm not sure what to use with the smaller bypass caps.
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Old 14th August 2007, 09:30 PM   #2
anatech is offline anatech  Canada
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Hi tkwou,
No wonder I couldn't find it! I'll move this to Solid State for you.

Quote:
I don't know if I should just bypass this with a good polypropolene or replace it with a good electrolytic too.
Do one thing at a time. Lose those tantalums! They are terrible things. Use any new electrolytic and bypass it with film types. You can get 1 uF film types.

Cyrus can still fix these on a "best effort" basis. I can as well. The information on these is contained in people's heads. No service manuals exist I don't think.

So, just use common sense. Replace the electrolytic types (age) and all tantalum types must go. Use your eye's and watch for bad solder joints due to age.

-Chris
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Old 15th August 2007, 06:48 AM   #3
tkwou is offline tkwou  Canada
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Thanks for your imput anatech

When I first examined the phono board and traced out the circuit I noticed the 220uF Tantalum used as the input coupling cap without any bypass. All other interstage coupling caps are bypassed. I read the original brochure for the 776/777 combo and it stressed the great pains they went through to minimize capacitor signature sound by using different brands and using bypass caps. The selection of passive component appears well thought out with 1% metal film resistors throughout. I don't know too much about that period of electronics (late 70's-early 80's). Were the tantalum caps thought to be "audiophile" caps at that time? It seem strange to me that they would have chosen the tantalum coupling cap unless they thought it sounded good.
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Old 15th August 2007, 05:08 PM   #4
anatech is offline anatech  Canada
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Hi tkwou,
Quote:
Were the tantalum caps thought to be "audiophile" caps at that time? It seem strange to me that they would have chosen the tantalum coupling cap unless they thought it sounded good.
Yes, they were in fashion,caused by many tweakers who didn't know anything. Manufacturers sometimes mus use parts that are thought to be good if they wish to sell product.

I consider this to be an unfortunate time in audio. Tantalums sounded better than the bad electrolytics possibly, but film caps clearly outclassed them. Time has corrected the situation.

One thing that did work in favour of tantalum caps in the position was the very low current leakage and small voltage applied across the capacitor. Tantalums fail to a direct short and this is not uncommon to see.

-Chris
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Old 15th August 2007, 06:48 PM   #5
tkwou is offline tkwou  Canada
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Thanks anatech

The tantalums in smaller values do I've found, make good decoupling caps on the voltage supply lines especially when bypassed by a smaller film cap. They just don't seem to sound good when you pass a signal through them.
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Old 16th August 2007, 06:10 AM   #6
anatech is offline anatech  Canada
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Hi tkwou,
Consider any tantalums across any supply line as a tiny little time bomb. Replace them before they short.

-Chris
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Old 21st September 2007, 04:51 PM   #7
tkwou is offline tkwou  Canada
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Well I finally got around to working on the preamp. While I had it apart I decided to do a complete overhaul. I change the tantalum input cap to an electrolytic bypassed with a polypropolene cap and I changed all the coupling caps to polyprolene matching all the original cap values to within 2% of the original part. Surprising it didn't make that big of a difference. The most improved is the high end which is little clearer, more defined but certainly no earth shattering improvement. The power supply is a 24V battery filtered through 10000uF Elna, split into two supply lines and then separetly filtered by 2200uF Elna and bypasses by a Rifa 4.7uF polyester cap. How long are filter caps good for? This preamp is atleast 25 years old. Are the electrolytics worth changing if the preamp sounds fine?
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Old 22nd September 2007, 01:27 AM   #8
anatech is offline anatech  Canada
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Hi tkwou,
Quote:
The most improved is the high end which is little clearer, more defined but certainly no earth shattering improvement.
That's it exactly. However, this is the very thing that changes a sound system from unlistenable to listenable. You may have a similar problem in your amplifier.
Quote:
How long are filter caps good for? This preamp is atleast 25 years old. Are the electrolytics worth changing if the preamp sounds fine?
Sometimes. Perhaps lower capacitance and an up to date regulator for each channel may make more of a difference.

-Chris
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Old 23rd September 2007, 05:06 PM   #9
tkwou is offline tkwou  Canada
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I'm cetainly a strong proponent of regulated power supplies with any AC ps for preamps and even poweramps up to the VA stage but how much benefit is there with regulating a battery ps for a low current drawing component like a preamp especially if all the gain stages are all constant current source loaded? The circuit and battery are rated at over 30 hours constant use without any degradation according to the design specs.
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Old 23rd September 2007, 07:18 PM   #10
anatech is offline anatech  Canada
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Hi tkwou,
Batteries can be noisy. Especially as the cell become discharged. I think smaller, good quality capacitors with a good regulator will have less noise.

-Chris
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