As a companion to the 700M nudies, here's theophile's 700C that I'm finishing up on. This one shall make a rather lengthy journey 'down under' in the near future. I should have taken pics before I started, but too late now. Oh well...
Anyway, for those unfamiliar with this preamp, it is a rather hard to find unit, and IMHO, probably the finest Japanese preamp made in the 70's, with a help of a few tweaks.
Kenwood originally designed this preamp to use nothing but polypropylene (or perhaps polyester) film capacitors in the signal path, but somewhere along the line (probably due to cost issues), the expensive film caps got axed and cheap non-polar electrolytics were installed instead. That's the bad news. The good news is that the traces on the PC boards still have the layout to allow mounting film caps as the Kenwood engineers originally intended, and ALL the inter-stage coupling caps are in the range of 0.47µf to 10µf, which leave the opportunity wide open to convert the amp to a 100% film capacitor signal path...no electrolytics. Doing this raises what is a very good preamp into a rarefied performance league that is unheard of for what you will pay for one of these.
Here's a pic of the top boards on a 700C that has been rebuilt, the original electrolytics replaced, and the interstage electrolytics removed and replaced with 250V Panasonic ECWF(B) Series metallized polypropylene capacitors...some of the best film caps I know of (these are the shiny rust-colored components). These are available in values up to 4.7µf. The tone amp uses a 10µf cap at the output, and for this I have chosen a 10µf 200V Auricap (the two big yellow suckers).
This is the only preamp I know that uses a soft-start circuit...similar to those used on very large (120WPC and larger) amplifiers and receivers. The stock power supply caps are 2200µf 80V, and as you can see I have replaced them with some computer-grade Cornell Dubilier 4700µf 75V caps. I've also replaced the original soft-start relay with a new one (an Omron LY2-DC24). Kenwood used a relay socket and made this easy.
Here's a pic of the bottom. Note that the tone controls are made from a multi-position switch with individual filter elements for each switch position rather than the standard potentiometer/Bandaxall filter as you see on units where cost-cutting is a primary concern. This is something you never ever see nowadays, but used to be standard practice on high-end gear till it just got too expensive. Open up some classic 60's Marantz gear made when Saul was in charge, and this is what you see. The labor costs to put one together was huge...must have taken a full work shift (or close to it) to solder one together .
The regulator supplies the +/-50V used by many stages, including the phono amp. Picture the minute swings up and down the Class-A AC load-line with 100V of range for the phono output (the smaller the swing in relation to the endpoints of the AC loadline, the more linear the amplification. High voltage rails put those endpoints a long ways away). Nice. The regulator also supplies +/-24V and +/-5V for other boards.
A nice feature that few will ever use is the ability to amplify a pair of microphones, one L and one R. Cool, whether it gets used or not.
Another nice feaure is the built-in headphone amp, something missing from many preamps (including the Pioneer Spec 1). It's a rather simplistic affair, and is incapable of driving headphones with impedances greater than 100 ohms to any worthwhile volume, but if you have a pair of 32 ohm Grado's or something you'll be pleased. There's also an adjustable headphone level control on the rear panel, but with most phones this will be turned up to maximum and volume adjusted only with the front panel volume control. The rear panel also has adjustable input impedance (600 ohm/30K Ohm/50K Ohm) for both phono 1 and phono 2, as well as two preamp outputs, and three Aux inputs, as well as the requisite two tape loops with full monitoring and dubbing capabilities.
All in all, a nice preamp. If you was fishing, you'll call it a 'keeper'.:thmbsp:
Anyway, for those unfamiliar with this preamp, it is a rather hard to find unit, and IMHO, probably the finest Japanese preamp made in the 70's, with a help of a few tweaks.
Kenwood originally designed this preamp to use nothing but polypropylene (or perhaps polyester) film capacitors in the signal path, but somewhere along the line (probably due to cost issues), the expensive film caps got axed and cheap non-polar electrolytics were installed instead. That's the bad news. The good news is that the traces on the PC boards still have the layout to allow mounting film caps as the Kenwood engineers originally intended, and ALL the inter-stage coupling caps are in the range of 0.47µf to 10µf, which leave the opportunity wide open to convert the amp to a 100% film capacitor signal path...no electrolytics. Doing this raises what is a very good preamp into a rarefied performance league that is unheard of for what you will pay for one of these.
Here's a pic of the top boards on a 700C that has been rebuilt, the original electrolytics replaced, and the interstage electrolytics removed and replaced with 250V Panasonic ECWF(B) Series metallized polypropylene capacitors...some of the best film caps I know of (these are the shiny rust-colored components). These are available in values up to 4.7µf. The tone amp uses a 10µf cap at the output, and for this I have chosen a 10µf 200V Auricap (the two big yellow suckers).
This is the only preamp I know that uses a soft-start circuit...similar to those used on very large (120WPC and larger) amplifiers and receivers. The stock power supply caps are 2200µf 80V, and as you can see I have replaced them with some computer-grade Cornell Dubilier 4700µf 75V caps. I've also replaced the original soft-start relay with a new one (an Omron LY2-DC24). Kenwood used a relay socket and made this easy.
Here's a pic of the bottom. Note that the tone controls are made from a multi-position switch with individual filter elements for each switch position rather than the standard potentiometer/Bandaxall filter as you see on units where cost-cutting is a primary concern. This is something you never ever see nowadays, but used to be standard practice on high-end gear till it just got too expensive. Open up some classic 60's Marantz gear made when Saul was in charge, and this is what you see. The labor costs to put one together was huge...must have taken a full work shift (or close to it) to solder one together .
The regulator supplies the +/-50V used by many stages, including the phono amp. Picture the minute swings up and down the Class-A AC load-line with 100V of range for the phono output (the smaller the swing in relation to the endpoints of the AC loadline, the more linear the amplification. High voltage rails put those endpoints a long ways away). Nice. The regulator also supplies +/-24V and +/-5V for other boards.
A nice feature that few will ever use is the ability to amplify a pair of microphones, one L and one R. Cool, whether it gets used or not.
Another nice feaure is the built-in headphone amp, something missing from many preamps (including the Pioneer Spec 1). It's a rather simplistic affair, and is incapable of driving headphones with impedances greater than 100 ohms to any worthwhile volume, but if you have a pair of 32 ohm Grado's or something you'll be pleased. There's also an adjustable headphone level control on the rear panel, but with most phones this will be turned up to maximum and volume adjusted only with the front panel volume control. The rear panel also has adjustable input impedance (600 ohm/30K Ohm/50K Ohm) for both phono 1 and phono 2, as well as two preamp outputs, and three Aux inputs, as well as the requisite two tape loops with full monitoring and dubbing capabilities.
All in all, a nice preamp. If you was fishing, you'll call it a 'keeper'.:thmbsp:
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:thmbsp: 
