How far can one go? KA-3750

[Rob DeFries]

After listening to the newly acquired KA-3750, I decided to give it a try to make it sound even better. I did get the idea that I was doing some overkill on the little girl. But who knows.



The unit was kinda dirty inside. Since you had to take the whole unit apart to get underneath the prints, I might aswell go the whole way.
The phono amp was a bit noisy, hiss and a lack of low end. So there was the start.


The amp is built around the 4558, frequently used in the green Ibanez TubeScreamer. Well, that's where it'll go.
I decided to take a more serious op amp, one of Burr Brown's OPA's. Along with the op-amp I changed out the cap's and resistors.


The ceramic caps were changed to styroflex, the low noise caps and bi-polars got Wima mkp's. The riaa mylar caps were reading 31nF in one and 39nF in the other channel, where 27nF should be right. I used 2 x 29nF mkp's for it.
For the op-amp I installed a 8 pin dip connector, so I can easily change them. I used a 2604AP, just because it was handy.



I left the two 120K resistors on the output for what they were, mainly because the were matched closely. They need to be changed too, later on. The print can use bigger end coupling caps if needed. There's a whole array of holes for various sizes of caps.



the 5 resistors and transistor are some kind of spike protection for the op-amp powerline. I don't think it'll do much harm to the audio signal, that's why I left it alone

Everything is back together again, I now want to listen to the phono stage. I'll let you know the outcome and show you the rest of my plans a.s.a.p.

 

[Rob DeFries]

This thing had some real, hard to remove, dirt on it's faceplate. Dang.
Sound from vinyl improved a lot, but I'm still not too happy about it.
Stereo placement is very nice, very wide and the low end goes LOW! Still it is not a clean sound, a bit grainy.

The upgraded Luxman L1 wins, with two fingers up its nose

 

[Rob DeFries]

I've been listening to the phono thing and I don't think it's worth to use the OPA.
I tried it with the 4558 again, and later with a NE5534. That one ist still in. Sounds as good as the OPA2604, for a lot less.

 

[Rob DeFries]

So, now the main amp will be subject to change.
I'm not going to touch the power transistors. Maybe in a later stage. The only two coupling caps will be swapped for the Wima 1uF caps.
Two electrolits in the power rail, 220uF, will be upgraded, the two big ones in the PS will go from 6800 to 8200uF.
There's no need to do more, because the girl doesn't have any further problems.
Opening it is straight forward. Pull the knobs off. Undo the four screws that hold the frontplate, two on top, two on the bottom and take the frontplate off.
With the unit upside down, undo the four screws which hold the cooling plate.
Put the amp back on it's feet and undo all the nuts of the pots, phones plug and on-off switch and take out the four screws that hold the tape monitor and loudness knobs. Undo the 4 countersunk screws (2 on each side) which holds the front chassis to the bottom. You can now take the front off, take the balance pot and phones plug out, put them aside. Same with the on-off switch.

Unsolder the black, blue and yellow wrap post out of the righthand side of the main print. Put them to the side.

 

[Rob DeFries]

This is how the 3700/3750 looks underneath. After so many years, it still looks good. No heat spots or bad solder joints. Quality!

One thing that I found odd was the presence of two 33uF/35V caps, soldered to the phones plug. The schematic says nothing about them two buggers, which I don't like there. Since the phones plug is also used to switch the speakers on and off, and also because the connectors have a different lay out than in the manual, it gave me te need to take it out. You can see one of the electrolits in the picture You can't see them if they're still in original trim.

Phones need their own amp and I don't want a possible scratch and distorting element in my signal way. So, in the end, I clipped the whole thing out. I soldered the orange and green speaker wires straight onto the print. Green where the blue wirewrap post was, orange where the yellow was.

 

[Rob DeFries]

An other thing that caught my eye is the use of two electrolits to form one bi-polar cap.
Used for the tone circuit in the feedback loop.
I'd noticed the four caps, clogged together, straight away.


Correct me if I'm wrong, but it seems still cheaper to use 2 electrolits than to use 1 bi-polar cap. I plumbed in two 4.7uF Nichicon Muse bi-polar Coupling caps.
Apart from the tone circuit and phones plug all the other changes are just swapping electrolits for new spiecies

Two 8200u power caps, 2 times 330u (220u) in the middle and some 100, 47 and 22u caps.
Putting the thing back together is easy too, but don't forget the 4 screws on both the push knobs.
It has been playing for two hours now. Let's listen if things have improved.

 

[Blue Shadow]

As to your series cap to make a non-polar, the cheap polar lytics are a nickel, the bipolars are 43 cents. Big difference. Kenwood was saving money with move for two reasons. 1) they used a lot of 10/25 lytics throughout their devices and they were very cheap and 2) using two polar caps kept them from having to mess with having the bipolar caps in stock.

 

[Rob DeFries]

Yep, I do understand. I was just surprised to see it.

 

[petehall347]

those caps are in plain sight in the schematic i have here ..c5 and c6 33uf 25v . should be series with resistors r1 r2 270 ohms ..
just studying schematic now to see if you may have lost phones output with them cut out .
edit ..yes phones wont work now as they couple the output to phones ..resistors for impedance matching ..caps for dc protection .

 

[Rob DeFries]

The 3750 is really relaxed to listen to. It can be a bit harsh at high volume, but at that level (volume at 3 o'clock position) I don't blame the little girl.
A little more highs would be nice. When the loudness button is depressed, it starts to sound like an old tube-amp. Massive lows, very warm but a bit over the top. Maybe experimenting with values gives a better outcome.
I do think the low frequencies benefit the most. Highs were a bit low and still are, although there's lots more detail than before.
Compared with my reverence low power amp, the Luxman L1, it came closer for detail and low end. A mighty fine amp.

Recap, yes! Upgrade phono op amp, yes, the NE5534 does a nice job for $ 0.60. Upgrade ps caps? I think it'll benefit. I had the caps handy anyway.
Maybe I'll up the power amp. No need for it at this moment. Loudness comes first.

 

[Rob DeFries]

Uhhh. I said I clipped the whole phones out......

 

[petehall347]

oh ..i thought you were improving it ... horses for courses i suppose ..if its never going to be used then why not .
i sort of stopped digesting the writing when there was mention of cutting caps out that arent in your manual .

 

[Rob DeFries]

those caps are in plain sight in the schematic i have here ..c5 and c6 33uf 25v . should be series with resistors r1 r2 270 ohms ..
just studying schematic now to see if you may have lost phones output with them cut out .
edit ..yes phones wont work now as they couple the output to phones ..resistors for impedance matching ..caps for dc protection .

They're not on my schematic. C5 and 6 are the coupling caps on mine.
I took the whole phones out, that's what I said..... I don't use phones anyway. And I don't like the extra switches.

 

[Rob DeFries]

oh ..i thought you were improving it ... horses for courses i suppose ..if its never going to be used then why not .
i sort of stopped digesting the writing when there was mention of cutting caps out that arent in your manual .

This manual says 3700/3750. Seems there are some differences in the schematics of the 3750. I have no idea what the real difference in these two amps is.

 

[petehall347]

that is the schematic for the 3700 .. go up one page
hold on i got that wrong ..both the same as it happens .. maybe it changed by serial numbers ..
all academic now as you cut it out ..

 

[Rob DeFries]

And the phones plug is an easy reversable operation. Just solder the blue and yellow wirewrap posts back in and solder the orange and green wire back onto the plug.

 

[Rob DeFries]

that is the schematic for the 3700 .. go up one page
hold on i got that wrong ..both the same as it happens .. maybe it changed by serial numbers ..
all academic now as you cut it out ..

I had a look and I did find an extra schematic on hifiengine. Just the schematic. That one has the caps. Could be serialnumber related, yes.

 

[petehall347]

i bet the caps were added at a later date when they had complaints of headphones setting on fire or similar . because of large dc hitting them when a fault occurred .

 

[Rob DeFries]

Fried headphones.
An other thing is: there is space left open and even the drawing for an adjustable pot on the print suggest that there was an intention for a bias or symm pot. I'll see what I can make of it, in the morning

 

[Rob DeFries]

VR5 and 6