Hi,
I have this amplifier that's quite in a bad shape (lots of bad contacts everywhere). My plan is to get rid of everything in it excepted the enclosure and power trafo and turn it into a diy amplifier.
I need to know the specs of the trafo in order to decide which DIY project is possible with it. It has 5 secondary wires on top that seem to fuel the main PCB with the output section and 3 wires that fuel another PSU board. The last one looks like a center tapped 15V but the other one is still mysterious, as the DMM say they are linked together...
Would anyone have any info about it ?
Thanx in advance !
Best regards.
Eric
I have this amplifier that's quite in a bad shape (lots of bad contacts everywhere). My plan is to get rid of everything in it excepted the enclosure and power trafo and turn it into a diy amplifier.
I need to know the specs of the trafo in order to decide which DIY project is possible with it. It has 5 secondary wires on top that seem to fuel the main PCB with the output section and 3 wires that fuel another PSU board. The last one looks like a center tapped 15V but the other one is still mysterious, as the DMM say they are linked together...
Would anyone have any info about it ?
Thanx in advance !
Best regards.
Eric
Hi, I was finally able to find the schematic here :
KENWOOD KA-94 SCH Service Manual free download, schematics, eeprom, repair info for electronics
With it I could find out that it's a CT secondary with a 52VAC and a 39VAC tap, that explained why I saw all 5 points connected.
What power amp project could I build with it ? I thought of ESP's 3A but I'm afraid that the voltage of my trafo is way too high... I could drop it with a regulator but that would be a huge waste of power, and the available current after the regulator might not be sufficient...
104VAC is also quite low for a decent tube project... 😉
Best regards.
Eric
KENWOOD KA-94 SCH Service Manual free download, schematics, eeprom, repair info for electronics
With it I could find out that it's a CT secondary with a 52VAC and a 39VAC tap, that explained why I saw all 5 points connected.
What power amp project could I build with it ? I thought of ESP's 3A but I'm afraid that the voltage of my trafo is way too high... I could drop it with a regulator but that would be a huge waste of power, and the available current after the regulator might not be sufficient...
104VAC is also quite low for a decent tube project... 😉
Best regards.
Eric
Too high for a P3A - but looks like a wellbuilt amp. Perhaps look at ostripper's designs, such as the "Honey Badger" amp. It should work well using the 39VAC taps to produce 55VDC supplies.
You could probably reuse the Kenwood's power transistors too. The outputs and drivers in that thing look good. Some pictures would help.
You could probably reuse the Kenwood's power transistors too. The outputs and drivers in that thing look good. Some pictures would help.
Hi ! Thanx a lot for your answers !
I live in Belgium actually ! 😛
Where did you see that info ?
Thanx for your advice ! I'll take a look at this project !
I found this picture on the web :
http://domainedelaconque.free.fr/ebay/33280.JPG
You're right I could keep the o/p transistors, but I think I'm gonna get rid of the PCB because I keep losing one side of the amp and it can sometimes be cured by gently pushing/hitting the back of the PCB with a drumstick. I tried to resolder a lot of things but without success. Sometimes the side of the amp reanimates by turning the volume fastly up, but those days that trick doesn't work anymore...
If anyone had a clue about what could be wrong in the amp I'd be glad to hear it, just for my personnal knowledges because I'm actually enthusiastic about building a DIY amp ! 😉
I actually have another question : I'm planning to feed the amp with an RME Multiface soundcard and through a Mackie HUI, that works at professional +4dBu level. Will I still need a preamp ? I'm planning to implement a symetrical input, as the rest of my chain is.
I found another project on ESP's site, the MOSFET power amp :
Project 101 - High Power, High Fidelity MOSFET power amplifier
That one could run at +/- 56V too. How do you think this one compares to the "Honey Badger" ? What's the point of using MOSFETS instead of bipolar output transistors ? I guess "soft clipping" is not someting you want to hear about in HIFI applications 😉
This is my first HIFI amp project, for now I mostly built studio stuff (mic pre, compressors...) and guitar stuff (loads of stompboxes and a tube amp coming soon), so please forgive my noob questions !
Best regards.
Eric
I live in Belgium actually ! 😛
Where did you see that info ?
Thanx for your advice ! I'll take a look at this project !
I found this picture on the web :
http://domainedelaconque.free.fr/ebay/33280.JPG
You're right I could keep the o/p transistors, but I think I'm gonna get rid of the PCB because I keep losing one side of the amp and it can sometimes be cured by gently pushing/hitting the back of the PCB with a drumstick. I tried to resolder a lot of things but without success. Sometimes the side of the amp reanimates by turning the volume fastly up, but those days that trick doesn't work anymore...
If anyone had a clue about what could be wrong in the amp I'd be glad to hear it, just for my personnal knowledges because I'm actually enthusiastic about building a DIY amp ! 😉
I actually have another question : I'm planning to feed the amp with an RME Multiface soundcard and through a Mackie HUI, that works at professional +4dBu level. Will I still need a preamp ? I'm planning to implement a symetrical input, as the rest of my chain is.
I found another project on ESP's site, the MOSFET power amp :
Project 101 - High Power, High Fidelity MOSFET power amplifier
That one could run at +/- 56V too. How do you think this one compares to the "Honey Badger" ? What's the point of using MOSFETS instead of bipolar output transistors ? I guess "soft clipping" is not someting you want to hear about in HIFI applications 😉
This is my first HIFI amp project, for now I mostly built studio stuff (mic pre, compressors...) and guitar stuff (loads of stompboxes and a tube amp coming soon), so please forgive my noob questions !
Best regards.
Eric
Using any mains transformer on a lower primary voltage reduces the secondary voltage by the same ratio.
As to your fault. It could be nothing more than tarnished speaker relay contacts. The problem dissappearing as you increase the volume is a classic symptom.
Edit... whats the point of MOSFET's ? Many many reasons why designers like them. Rugged, cheap, easy to drive, extended hf response compared to bipolar. Lateral FET's are my preferred choice.
As to your fault. It could be nothing more than tarnished speaker relay contacts. The problem dissappearing as you increase the volume is a classic symptom.
Edit... whats the point of MOSFET's ? Many many reasons why designers like them. Rugged, cheap, easy to drive, extended hf response compared to bipolar. Lateral FET's are my preferred choice.
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Not really qualified to speak for certain on MOSFETs but laterals are a lot more expensive and harder to get than good audio power transistors I think. Rod's circuits are usually very good. Are you looking to buy a premade PCB or are you happy to make your own?
Preamp, probably not - just a volume pot, though I would still put a unity gain buffer in front of it so that the preceding stage doesn't see a changing impedance due to the volume control.
Hm, the picture has one pair of output transistors in a TO-3P package per channel. I guess they are using power supply sag to make sure their safe operating area isnt exceeded. I'd recommend two pairs of TO247/TO264 package devices per channel at least if you build your own amp in there.
Preamp, probably not - just a volume pot, though I would still put a unity gain buffer in front of it so that the preceding stage doesn't see a changing impedance due to the volume control.
Hm, the picture has one pair of output transistors in a TO-3P package per channel. I guess they are using power supply sag to make sure their safe operating area isnt exceeded. I'd recommend two pairs of TO247/TO264 package devices per channel at least if you build your own amp in there.
In Belgium we're actually running on 230V
Hmm you're right I might try to temporarly short the relay to see if that solves the problem ! Then I'd keep the amp as it is and maybe build another amp then later for fun 😉
The relay is here to make sure no DC offset is present on the output, right ? I'll check the output signal with a scope/DMM before conecting a speaker.
Do you mean that the output transistors are oversized compared to the PSU/heatsinks ?
Best regards.
If the relay comes apart (a good many have a top that just pops off) then just pull some paper or card soaked in WD40 through the contacts to clean them.
Although the relay protects from DC offset etc it can also be used as a delay to prevent switch on and off noises that most amps make.
Although the relay protects from DC offset etc it can also be used as a delay to prevent switch on and off noises that most amps make.
Hi,
I changed the relay and now... it works perfectly ! I just resoldered a few cold joints here and there on the cinch input PCB, cleaned the pots and now the amp is working like new !
Finally this amp won't be the home for a new DIY amp but at least it gave me clues for an eventual project of that kind.
Thanx a lot for your advices and tricks ! If I had suspected the relay earlier I'd have repaired that amp years ago !
Best regards.
Eric
I changed the relay and now... it works perfectly ! I just resoldered a few cold joints here and there on the cinch input PCB, cleaned the pots and now the amp is working like new !
Finally this amp won't be the home for a new DIY amp but at least it gave me clues for an eventual project of that kind.
Thanx a lot for your advices and tricks ! If I had suspected the relay earlier I'd have repaired that amp years ago !
Best regards.
Eric
Thank you for your help !
By the way I noticed something annoying : I intend to use that amp to drive speakers to provide me an alternative monitor for my home studio. My main monitors are Dynaudio BM6A active studio monitors. When I switch the amp on, I hear a "pop" in the Dynaudios. The pop is loud and has lots of treble in it, just like a perfectly square impulse. I guess it's the sudden current peak the amp draws when it switches on that causes the mains to sag fastly.
The monitors are both fed by the monitoring section of my Mackie HUI remote. The phenomenon is much more dramatic when the Mackie is on : when only the Dynaudios are on the "pop" is much quieter. When I plug headphones to the Mackie to hear it contribution to the "pop", it is also present but quiet.
Are there any ways to get rid of that problem ? It also occurs when I switch light on/off, but there it's much quieter than with the amp. Is it possible to add some filtering to the mains, with big 350V filter caps or big inductors like in guitar tube amps ?
I run the Mackie HUI through a 230 to 115VAC trafo, I don't know wether that can have an influence (it's an american 110V model). The trafo is a Hammond 175B-NA auto transformer.
To give you an idea of the gear involved :
http://cachepe.zzounds.com/media/quality,85/HUITop9-17-1b07e8b60f3c56e05498eb432c359156.jpg
http://www.prodj.com.ua/images/img2_id/20100624124317.jpg
Hammond Mfg. - "AUTO" Plug In - Step Down - Line Transformers - (175 Series) (input plug changed to EU)
Best regards.
Eric
By the way I noticed something annoying : I intend to use that amp to drive speakers to provide me an alternative monitor for my home studio. My main monitors are Dynaudio BM6A active studio monitors. When I switch the amp on, I hear a "pop" in the Dynaudios. The pop is loud and has lots of treble in it, just like a perfectly square impulse. I guess it's the sudden current peak the amp draws when it switches on that causes the mains to sag fastly.
The monitors are both fed by the monitoring section of my Mackie HUI remote. The phenomenon is much more dramatic when the Mackie is on : when only the Dynaudios are on the "pop" is much quieter. When I plug headphones to the Mackie to hear it contribution to the "pop", it is also present but quiet.
Are there any ways to get rid of that problem ? It also occurs when I switch light on/off, but there it's much quieter than with the amp. Is it possible to add some filtering to the mains, with big 350V filter caps or big inductors like in guitar tube amps ?
I run the Mackie HUI through a 230 to 115VAC trafo, I don't know wether that can have an influence (it's an american 110V model). The trafo is a Hammond 175B-NA auto transformer.
To give you an idea of the gear involved :
http://cachepe.zzounds.com/media/quality,85/HUITop9-17-1b07e8b60f3c56e05498eb432c359156.jpg
http://www.prodj.com.ua/images/img2_id/20100624124317.jpg
Hammond Mfg. - "AUTO" Plug In - Step Down - Line Transformers - (175 Series) (input plug changed to EU)
Best regards.
Eric
You may want to check that the relay circuit is working. There should be a delay on powerup of a few seconds. If the relay immediately engages, there's a problem.
You could also be hearing the spark across the power switch contacts - the schematic suggests there is no "snubber" capacitor across the power switch S1. Solder a 47nF-100nF 275V X2 rated (the X2 rating is important!) across S1 and it should help. If you are looking for a cheap source for one of those, pull apart a dead PC power supply 🙂
You could also be hearing the spark across the power switch contacts - the schematic suggests there is no "snubber" capacitor across the power switch S1. Solder a 47nF-100nF 275V X2 rated (the X2 rating is important!) across S1 and it should help. If you are looking for a cheap source for one of those, pull apart a dead PC power supply 🙂
As jaycee says really...
check that the relay takes a few seconds to pull in when mains is switched on.
Light switches (anything switching) can cause interference in a couple of ways. It can be a burst of RF energy that is picked up and "demodulated" by the input stages of the amp, or it can generate a pulse of interference in any nearby wiring.
Maybe try the amp in isolation with only speakers and nothing else connected to begin with.
check that the relay takes a few seconds to pull in when mains is switched on.
Light switches (anything switching) can cause interference in a couple of ways. It can be a burst of RF energy that is picked up and "demodulated" by the input stages of the amp, or it can generate a pulse of interference in any nearby wiring.
Maybe try the amp in isolation with only speakers and nothing else connected to begin with.
Hi,
I think my post wasn't really clear. When I switch the Kenwood amp on with speakers, the switching is totally silent, that is to say the relay is playing its role correctly.
The amp is in the same room as my Dynaudio BM6A studio monitors. When I switch the Kenwood on while the Dynaudio are already on, I hear that loud "pop" in the Dynaudio. So nothing wrong with the amp itself, but there might be something wrong in the mains cabling of my flat, that "transforms" the sudden current draw of the amp switching on to a "pop" in my Dynaudio !
Could also be that the mains input ibn the Dynauio isn't filtered/isolated correctly...
Do you have any clues about how to cure it ?
Best regards
Eric
I think my post wasn't really clear. When I switch the Kenwood amp on with speakers, the switching is totally silent, that is to say the relay is playing its role correctly.
The amp is in the same room as my Dynaudio BM6A studio monitors. When I switch the Kenwood on while the Dynaudio are already on, I hear that loud "pop" in the Dynaudio. So nothing wrong with the amp itself, but there might be something wrong in the mains cabling of my flat, that "transforms" the sudden current draw of the amp switching on to a "pop" in my Dynaudio !
Could also be that the mains input ibn the Dynauio isn't filtered/isolated correctly...
Do you have any clues about how to cure it ?
Best regards
Eric
You need to isolate the problem if possible. Try the Dynaudios with no inputs (the input leads removed) and see if they still make the noise. If they do then its either being radiated as an RF burst or its entering via the mains.
There is always the possibility of a problem with the property wiring, maybe an earthing issue somewhereor a high impedance earth (in the mains wiring).
When you switch a light on/off the circuit is normally just a series switch in the live wire so the fact it still makes the noise when a light switch is thrown maybe suggests a radiated issue.
If there is any doubt over the wiring then it needs to be checked. Could you try the speakers in another room/outlet or even property.
Adding snubbers across things that cause the problem (if that did actually help) isn't perhaps the answer as there is an underlying reason for the issue.
There is always the possibility of a problem with the property wiring, maybe an earthing issue somewhereor a high impedance earth (in the mains wiring).
When you switch a light on/off the circuit is normally just a series switch in the live wire so the fact it still makes the noise when a light switch is thrown maybe suggests a radiated issue.
If there is any doubt over the wiring then it needs to be checked. Could you try the speakers in another room/outlet or even property.
Adding snubbers across things that cause the problem (if that did actually help) isn't perhaps the answer as there is an underlying reason for the issue.
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