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Old 21st November 2012, 01:22 AM   #1
bgruhn is offline bgruhn  United States
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Default Where to from here? Preamp?

Have just to put finishing touches on LT phono arm. Am enjoying it immensely and thinking about upgrading some of the circuitry it plays into. Being the nearly ultimate cheapskate I am, my system presently consists of a pair of home brew Isobarik speakers driven by an old but very good sounding Kenwood KR 3130 receiver. My LP record collection is the primary source of music, played by an Ortofon OM cartridge on the just built LT arm. All of this lives in a rather small living room, carpeted, some upholstered furniture and lots of plants on one of the long walls. Reflections are not a major issue and the room is not an anechoic chamber. Now to the issue of this thread. There are a bunch of interesting pre-amps discussed in this forum. Some are most applicable to neophyte audiophiles and others look to be for the golden ears set. I'm neither. Without doubt I am in the golden age set. Now assuming that the phono front end from the 1970s Kenwood could be improved on, my question is what magnitude of change could I reasonably expect by feeding the phono signal to one of the better pre-amps discussed in this DIY Audio forum and from there to the aux input of the KR 3130 or any other receiver electronics. Throwing all this stuff out and starting over is not an option. Your opinions and experiences with similar equipment are requested please. I do have a 100wpc Eumig m1000 power amp, but no preamp for it presently. That plus a good pre-amp could easily be pressed into service. Thanks for any and all suggestions or comments.

BillG
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Old 22nd November 2012, 07:01 AM   #2
h_a is offline h_a  Europe
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Quote:
Ortofon OM
Cheapskate rule #1 for moving magnet cartridges - check its proper termination. Every moving magnet cartridge needs a proper capacitance (say 300pF) as load for a 'neutral' frequency response.

When this sounds new to you, it is the first place to start as it's almost free (needs only a few pF ceramics) and provides maximum value. As starting point check the manufacturer recommendation on their website and don't forget to subtract the usual 100pF of the cabling. You can experiment then a bit, 50pF are clearly audible!

Enjoy!
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Old 22nd November 2012, 02:39 PM   #3
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I'm another LP collector, and now the people that were holding all the good stuff (Mercury Living Presence!, RCA Red Seal, Colombia) are dieing off and they are $.50 each at Goodwill and Salvation Army.
I've never needed anything better sounding than the $40 Dynaco tube PAS2 (with new electrolytic caps and selenium rectifier replaced with silicon) but the volume control is impossible to get and the replacement I do have has to be turned with vice grip pliers. Also it uses 100 watts. And a PAS2 or 3 will cost you probably $200 unrestored, there are only so many of them.
So I bought a disco mixer for $15, and spent 9 month upgrading it where it sounds good. Not quite to the PAS 2 hum level, but with 6 input pots I don't have to walk around and switch the selector and fiddle with the volume at all to switch from CD to FM radio to LP. www.diyaudio.com/forums/analog-line-level/164102-improving-disco-mixer-mid-fi-performance.html is the saga of the improvement.
Perhaps your receiver could use a few cheap tricks to improve its sound. Post the front end part of the schematic and we'll take a look. Things like replacing the $.12 Riaa roll off capacitor with a $2.50 10X one and cutting the value of the paired resistor by 10 for less hiss. That is one I still have to do to my disco mixer, but I'm having too much fun listening to it to stop doing my other projects.
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Old 22nd November 2012, 05:05 PM   #4
bgruhn is offline bgruhn  United States
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Happy T day indianajoe (and to the rest of the forum members)

Your saga of the PAS volume control got me thinking. Years ago I was in the radio/tv service business and repaired as well as replaced hundreds of volume controls. The vice gripe knob sounds a bit heavy. I think you can cure the sticky pot. Replacement may well be impossible as it is a dual pot with attached switch. Probably has a loudness tap as well. The hard turning most likely comes from lubricant that has gummed up something fierce. First try if you haven't done so already squirting acetone or lacquer thinner or MEK into the shaft to penetrate the gum in the bushing. Keep squirting and turning. The probability is high that you can free it up that way. Now if the sticking part is in the bushing for the rear element, you will have to disassemble the unit. With a little care, you can do this. Then clean the shafts and bushings thoroughly with solvent and check to see if they rotate freely. If so dry them put a very thin layer of light oil, or lubriplate on the shafts and put it back together. If after cleaning them out, if they are still too tight in the bushings, polish the shafts down for a smooth fit using #400 or #600 wet&dry paper. It is just possible that the switch could be sticking, but I doubt it. If all else fails, get a Stilson wrench.

You mention your replacement uses 100 watts. I think this is a rating of the current carrying capacity of the switch. A 100 watt potentiometer would be a wirewound unit and huge. Never get it into the case and if dissipating 100 watts would burn up the pre-amp in jig time.

I like your idea of using the mixer board. Good thinking. When I locate a schematic I'll post the front end circuit for the receiver. So far I'm quite pleased with the old thing and haven't identified any problems that really need fixing. Just thinking that after 30 years it must be suffering something.

Happy scrounging for good LPs,
BillG

Quote:
Originally Posted by indianajo View Post
I'm another LP collector, and now the people that were holding all the good stuff (Mercury Living Presence!, RCA Red Seal, Colombia) are dieing off and they are $.50 each at Goodwill and Salvation Army.
I've never needed anything better sounding than the $40 Dynaco tube PAS2 (with new electrolytic caps and selenium rectifier replaced with silicon) but the volume control is impossible to get and the replacement I do have has to be turned with vice grip pliers. Also it uses 100 watts. And a PAS2 or 3 will cost you probably $200 unrestored, there are only so many of them.
So I bought a disco mixer for $15, and spent 9 month upgrading it where it sounds good. Not quite to the PAS 2 hum level, but with 6 input pots I don't have to walk around and switch the selector and fiddle with the volume at all to switch from CD to FM radio to LP. www.diyaudio.com/forums/analog-line-level/164102-improving-disco-mixer-mid-fi-performance.html is the saga of the improvement.
Perhaps your receiver could use a few cheap tricks to improve its sound. Post the front end part of the schematic and we'll take a look. Things like replacing the $.12 Riaa roll off capacitor with a $2.50 10X one and cutting the value of the paired resistor by 10 for less hiss. That is one I still have to do to my disco mixer, but I'm having too much fun listening to it to stop doing my other projects.
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Old 23rd November 2012, 04:24 AM   #5
bgruhn is offline bgruhn  United States
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[QUOTE=indianajo;
Perhaps your receiver could use a few cheap tricks to improve its sound. Post the front end part of the schematic and we'll take a look. Things like replacing the $.12 Riaa roll off capacitor with a $2.50 10X one and cutting the value of the paired resistor by 10 for less hiss. projects.[/QUOTE]

Hi indianajoe,
I found a schematic of the Kenwood KR-3130 receiver. It is a pdf file and I couldn't select just the preamp circuit. Then this forum wouldn't accept the full diagram file. Way too big. So I posted the full schematic on a website of my own where you can view it with Adobe Reader and easily adjust the size for good viewing. Go to this URL please.
www.gis.net/~bgruhn/audiodiy. Then select the file KR3130.PDF. It didn't want to work with a one step url (wierd!)
Thanks for any inputs on tweaking this circuit.

BillG

Last edited by bgruhn; 23rd November 2012 at 04:32 AM.
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Old 23rd November 2012, 07:00 AM   #6
h_a is offline h_a  Europe
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Hi BillG!

please help me understanding that - my hint with proper cartridge termination is exactly what you wanted (keep the existing gear, major improvement at no cost etc) and yet it appears nobody's interested

Is it because you're keen on building something new or because I wasn't fully clear what I meant?

I see that happening now and then (people prefering to build new preamps instead of first getting their cartridge working optimally), so it would be great if you could drop me a line! Thanks!
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Old 23rd November 2012, 08:36 AM   #7
bgruhn is offline bgruhn  United States
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Quote:
Originally Posted by h_a View Post
Hi BillG!

please help me understanding that - my hint with proper cartridge termination is exactly what you wanted (keep the existing gear, major improvement at no cost etc) and yet it appears nobody's interested

Is it because you're keen on building something new or because I wasn't fully clear what I meant?

I see that happening now and then (people prefering to build new preamps instead of first getting their cartridge working optimally), so it would be great if you could drop me a line! Thanks!
Hi H_A,
Sorry about the confusion. It is clear that I didn't really make myself clear in the first post. You made yourself clear that there was a real improvement which could be gotten by achieving optimum cartridge match, but not what those improvements are. What I have in this old receiver is something that maybe needs matching and maybe doesn't need it. If it doesn't, no improvement can be achieved from that effort. Maybe it needs to have all the electrolytics replaced. Maybe the basic circuit isn't capable of the performance I would like to have. wringing the best out of this machine could be a real bootstrap operation. In the meantime I'm using it and enjoying it.

I mentioned owning a Eumig M1000 power amplifier. I really liked that one when I was last using it. But I don't have any preamp for it at present. Having a nice modern preamp with all new components could put the M1000 to work again and provide a reference to judge the receiver against.

So what I'm looking for is lots of information to plan where I'm going next. Your suggestion about the cartridge matching is definitely something I will do when the time comes. Given that I intend to stay with mm cartridges it certainly will be worth doing even if no modifications are needed.

Thanks for your rapid response and pardon my less than rapid reply. Since you and the Eumig are both Austrian, do you have any information or feelings (positive or negative) about the M1000 power amp?

Regards,
BillG
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Old 23rd November 2012, 04:57 PM   #8
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My RA88a disco mixer had 100 pf ceramic caps to case ground at the RCA input jacks. This both shorts out RF input to ground, and approaches what HA says about proper MM cartridge termination, 100 pf of the cables and 100 pf at the jack. What I did to improve the disco mixer was separate analog ground of the PWB from the case ground by installing the RCA jacks on O-rings, then moving the 100 pf cap leads to a terminals strip connected to the ground of the PWB. Improved hum.
The pdf file is right at the limit of what my computer will accept without locking up, so I didn't get a very good look. But, there doesn't appear to be a 100 pf cap to either analog or case ground at the input jack for phono on the Kenwood. Also, there are lot of electrolytic caps in the front end of the preamp. 3.3 uf, 100 uf, 10 uf. These likely are not the original value at operating temperature at 30 years, and should be replaced. John Curl and some other experts on the SS forum believe in the accuracy of polyprophylene dielectric caps. These can be bought up to 10 uf and if they will fit in the place, should be tried. Where they wouldn't fit in my organ, and at the input of my ST120 amp, I used Aerovox brand gold 50 V CPO ceramic caps. Mr. Curl would have a cat over that, but I like the resulting sound way better than the tantalum caps I took out of input the ST120- which had low level popcorn noise from the day I bought them in 1990. I think the 50v rating on 1.6 v signals of the CPO ceramic caps swamps the known non-linearity of ceramic caps with a very long capacitance versus voltage curve. but the Aerovox gold caps are about $5 each, are not something a commercial amp designer would use.
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Last edited by indianajo; 23rd November 2012 at 05:00 PM.
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Old 26th November 2012, 09:07 AM   #9
dangus is offline dangus  Canada
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DJ mixers, even comparatively good ones, don't usually have the greatest phono stages. It's typically a single opamp stage which tends to compromise RIAA accuracy and stuff. Sometimes things which on the surface ought to be good, like UREI's outboard phono preamp turns out to be a cheap transistor preamp stage with a balanced line driver added.

When Audioxpress did a review of budget phono preamps a while back, the hands down winner was the Hagtech Bugle. It appears that some awful people in China have copied the Bugle circuit and are offering them on eBay at a very tempting price. Search for RIAA OPA2134; you'll most likely win the auction at the starting price. In a pinch you could run it from a couple of 9V batteries, but my preference would be a good regulated supply, possibly using the eBay SKU153006 board.

If the power amp has enough gain, you might get away with a simple passive "preamp" after the phono preamp; a decent volume pot and a selector switch, assuming you listen to other sources besides vinyl.

On the other other hand, Hafler DH-101 preamps are common and sell on eBay for reasonable prices. And diligent searching of local sources of used audio gear may turn up something respectable.
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