Which OpAmp for Kenwood KT-7500 tuner mod?

[tusntuk2]

There have been a couple of great threads about modding the KT-7500 to improve the sound. I'm not looking for the FM tuner mods to the filters and such, just looking to improve the sound. I've done several Kenwood restorations now, and the last success (KA-7100 - wow!) has prompted me to try modding the KT-7500 tuner.

So, in trying to mod a KT-7500 with the popular upgrades - there was lots of old discussion about the 2604 opamps, but they are long obsolete. I'm a basic replace-the-caps and tx's guy, and I have not yet tackled opamps. Will something like an OPA2134 work here? I also selected a socket for it, (again not sure of what I'm doing!) it's Mouser 110-41-308-41-001000. Am I on the right track?

As usual, you guys are the best! Thanks in advance!

 

[tusntuk2]

Hey great, thanks rj! Yes, I was planning on the recap. that's what I've been doing with the amps and receivers. Each time I am amazed at the difference, but it is so hard to explain. I got to play the last modded amp (7100) for my techie musician friend, and I turned it on really low and quiet. He twisted and turned his head (kind of like my puppy...) and listened and he said, "where did you get THAT amp?" Uh-huh! So now I want a clean KT-7500 to go with it.

Thanks for the tip on the opamp. The threads, and parts, are getting old, like me! But I don't know anything about opamps yet. First the recap, then consider some of the wire changes, then the opamp if I get that far. Sure 'preciate it!

 

[Grenadeslio]

Hey great, thanks rj! Yes, I was planning on the recap. that's what I've been doing with the amps and receivers. Each time I am amazed at the difference, but it is so hard to explain. I got to play the last modded amp (7100) for my techie musician friend, and I turned it on really low and quiet. He twisted and turned his head (kind of like my puppy...) and listened and he said, "where did you get THAT amp?" Uh-huh! So now I want a clean KT-7500 to go with it.

Thanks for the tip on the opamp. The threads, and parts, are getting old, like me! But I don't know anything about opamps yet. First the recap, then consider some of the wire changes, then the opamp if I get that far. Sure 'preciate it!

I can understand your new found liking of Kenwood gear, I've been a fan since 1978 when I bought a new KA-7100 along with a KT-6500.

Yes the 7100 is an awesome amp, let it and the tuner go in 91, always missed it. I replaced them a couple of yrs ago with the KA-7300 and KT-7300.

 

[kevzep]

I rehabbed a KT7500 just last week.
Yes the OP amp they used in this model is possibly the worst on the planet.

It greatly improves the audio quality getting rid of that op amp. I used a 5532 as that is what I had on hand, the 20mhz version, and this made a huge difference. If you wanted to go exotic you could use a LM4562, they are as high end as you can get, and twice the bandwidth and 4 times the slew rate of the op amp suggested a few posts earlier......

BUT you also need to upgrade several coupling caps as well, as in stock form the 7500 has very poor low end.
Namely C45 and C46, which are those brown caps, standard they are 0.15µf, go up to 1µf using a nice stacked film cap.
I also upgraded a lot of the power supply caps too, I fitted a 2,200µf for the main cap, and the others in the power supply circuit, pretty much doubled the values.
There was a few caps I changed around the detector chip as well, I can't remember off the top of my head what they were changed to...

Then I have it a decent alignment, and I have to say it sounds very very nice now.

 

[tusntuk2]

Great to hear from you kevzep! Thanks for the tip for the LM4562. It's funny, I am a noob with opamps, I've only worked on tubes and analog so far. I was put off by the crazy opamp prices, some over $100 (US) so you don't go flippin a few in there to see what sounds best. Some suggestions come in around $5, and now your suggestion - wait, it's only $1.64! Guess I need to understand slew rate and bandwidth more!

I am a retired bass player, so the bottom end is real important to me. I was planning on upping C45,46 to .47uf film, and considering the 'Ray's EQ mod' that used a 1uf in series with a 10k resistor. Got to hear that bass! I've had success boosting a few in the KA-3500's and 7100's I've worked on.

Thanks for your help!

 

[tusntuk2]

What adjustments are needed to accomodate the LM4562? Seems like it might need reduced voltage, but the datasheet schematic shows a bit different connection than the 4558, so I wonder if I need to change anything to use the new opamp.

 

[kevzep]

What adjustments are needed to accomodate the LM4562? Seems like it might need reduced voltage, but the datasheet schematic shows a bit different connection than the 4558, so I wonder if I need to change anything to use the new opamp.

Bass is very important!! and I'm a guitarist!!

No modification necessary, just drop it in, same pinout....Voltage is fine.

What op-amps cost 100 bucks? I've never heard of that before, they are always around $2-ish...

I think you'll like that tuner once you have those things sorted...

 

[tusntuk2]

Great, thanks again kev. I'll try the 4562. And learn something about opamps too. I've got a bass amp, an early SWR, that's a real hi-fi bass amp. Its got a LOT of ic's, and it needs help. But thats another topic, hey?

Oh yeah, Rock Bottom!

 

[tusntuk2]

hey wow - what time is it there? Its 3:35pm here in central usa

 

[kevzep]

hey wow - what time is it there? Its 3:35pm here in central usa

Its 9:33am Monday morning...

 

[turboyam]

Great, thanks again kev. I'll try the 4562. And learn something about opamps too. I've got a bass amp, an early SWR, that's a real hi-fi bass amp. Its got a LOT of ic's, and it needs help. But thats another topic, hey?

Oh yeah, Rock Bottom!

Say, i know it's been a while but, did you try the updates? the LM4562's?

 

[tusntuk2]

hey turbo - yeah, it's been a long time and a few more units under the bridge since then. I went back through the notes and found this:

"NJM 5532 - good, clean
LM 4562 - best balance of bass/treble
NJM 2068d - OK, softer "

These notes based on my ears only. I try to keep notes on the work I do, because I'm getting old and I don't remember, and because there's too much to know anyway. In the notes on the mod for the 7500 I spent much more time discussing changes to the de-emphasis making more noticeable differences to the tone than the op amps.

It's funny, but sometimes the mods we do don't have an obvious "Oh, I can hear that" difference, but rather more of an opposite effect by cleaning up a signal, or reducing background noise or distortion, or providing power more quickly when demanded. Then it just sounds better, but you can't tell exactly why. I describe it as cleaning the windshield - you don't really see the blue film until you start to clean it off, then it becomes more apparent in its absence. The mods on the 7500 were like that, nothing very obvious, just cleaner, more easily detailed in your ear. The quality of the sound improved overall, it was more musical, easier and more rewarding to listen to. Was it the 4562? Hell if I know!

Carry on and be safe!

 

[turboyam]

hey turbo - yeah, it's been a long time and a few more units under the bridge since then. I went back through the notes and found this:

"NJM 5532 - good, clean
LM 4562 - best balance of bass/treble
NJM 2068d - OK, softer "

These notes based on my ears only. I try to keep notes on the work I do, because I'm getting old and I don't remember, and because there's too much to know anyway. In the notes on the mod for the 7500 I spent much more time discussing changes to the de-emphasis making more noticeable differences to the tone than the op amps.

It's funny, but sometimes the mods we do don't have an obvious "Oh, I can hear that" difference, but rather more of an opposite effect by cleaning up a signal, or reducing background noise or distortion, or providing power more quickly when demanded. Then it just sounds better, but you can't tell exactly why. I describe it as cleaning the windshield - you don't really see the blue film until you start to clean it off, then it becomes more apparent in its absence. The mods on the 7500 were like that, nothing very obvious, just cleaner, more easily detailed in your ear. The quality of the sound improved overall, it was more musical, easier and more rewarding to listen to. Was it the 4562? Hell if I know!

Carry on and be safe!

Thank you! I'll try this for sure!.

 

[Salty Guy]

I'm reading your thread as I research op amps... I'm a total noob... but I have a decent of collection of units I want to learn to work on. I think my Kenwood system is the one I want to get up and running the most. I have a KA 9100 that has power on issues, buzzing clicking I think in the relay, so I don't even know how it performs beyond that. Still I found me a minty fresh KT 7500 to match the beautiful face of the KA 9100. I can't wait to know enough to be able to work on them.

Still the op amp I'm looking to replace is on my Marantz 3800 pre-amp. Your thread pleasantly side tracked me. So I thought I'd pop in. If you interested, I attached pic of my Kenwood amp and tuner.

 

[jegastell]

Rehabilité un KT7500 la semana pasada.
Sí, el amplificador OP que usaron en este modelo es posiblemente el peor del planeta.

Mejora enormemente la calidad del audio al deshacerse de ese amplificador operacional. Utilicé un 5532 porque era lo que tenía a mano, la versión de 20 mhz, y esto marcó una gran diferencia. Si quisieras algo exótico, podrías usar un LM4562, son de la gama más alta posible, y tienen el doble de ancho de banda y 4 veces la velocidad de respuesta del amplificador operacional sugerido en algunas publicaciones anteriores...

PERO también es necesario actualizar varias tapas de acoplamiento, ya que en stock el 7500 tiene un extremo inferior muy pobre.
Es decir, C45 y C46, que son esas tapas marrones, estándar son 0,15 µf, suben a 1 µf usando una bonita tapa de película apilada.
También actualicé muchas de las tapas de la fuente de alimentación, instalé 2200 µf para la tapa principal y las demás en el circuito de fuente de alimentación prácticamente duplicaron los valores.
También cambié algunas tapas alrededor del chip detector, no puedo recordar en qué se cambiaron...

Entonces tengo una alineación decente y debo decir que ahora suena muy, muy bien.

hola,tengo kenwood kt 7500 y me gustaria poder restaurarlo y hacerle cambio

 

[Blue Shadow]

hola,tengo kenwood kt 7500 y me gustaria poder restaurarlo y hacerle cambio

download the service manual: https://www.manualslib.com/manual/1008169/Kenwood-Kt-7500.html
Then decide what you want to. Most restorations are just a replacement of the electrolytic capacitors. Others go further. Seems the 7500 could benefit from a chip upgrade.