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AUDIO FORUMS >> General Discussion and Support >> Decware ZMF OTL Distortion
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Message started by ck293 on 09/02/24 at 00:35:35

Title: Decware ZMF OTL Distortion
Post by ck293 on 09/02/24 at 00:35:35

I recently acquired a zmf decware otl amp. I had some noisy tubes and Zach gave me replacement tubes. I sent out the old tubes and last night I noticed some noticeable bad distortion in some songs. I have to imagine that it's something with the new tubes. I used spare 5u4g and 6n1p tubes and it didn't fix it. I ordered a set of new 6n6p tubes. My guess is it could be that. Can bad driver/power tubes cause noticeably bad distortion?

Title: Re: Decware ZMF OTL Distortion
Post by Donnie on 09/02/24 at 03:27:40

Did the amp distort with the noisy tubes?
If it didn't distort before you changed tubes, that kinda points the finger at the new tubes.

Are you using it as a headphone amp or as a preamp? I remember seeing a earlier post before about using it as a preamp. Just curious.

Title: Re: Decware ZMF OTL Distortion
Post by will on 09/02/24 at 14:42:34

It does sound like replacement power tubes made the distortion change. Still...not familiar with this amp except from a ZMF video, but I wonder if you have played with the gains in and out near the RCAs to see if that helps solve the distortion?

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/02/24 at 15:07:38

I'm fairly certain it's the power tubes. I have used it as a preamp for my speakers and as headphone amp. I compared against my Don sachs tube preamp on same song and it sounded fine.

Title: Re: Decware ZMF OTL Distortion
Post by will on 09/02/24 at 16:26:27

Guessing the replacement tubes were well tested, especially since you had an issue with noise with the originals? If so, and if both channels are distorting, the chances of it being the new tubes get considerably slimmer. One maybe, but a pair seems pretty unlikely. Also that is it is not on all recordings made me think of input gain potentially being borderline too hot, showing up only on hot recordings or hot passages of those? The output tubes being the final signal maker/amplifier, everything before affects them, including the input tube, and those variable also electronically. So if new/tested replacement tubes, and if distorting on both channels and just on just some recordings, the output tubes seem less likely to me.... for troubleshooting, making it worth a look at too much input gain for the particular tube set you are using.

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/02/24 at 16:32:26

Thanks for the reply. I'm not confident they were tested that well. The replacement input tube i received still has a slight hum. When using one of my tubes from my zen torii...there is no hum at all. I have ordered a new set of input and power tubes from tube store. I will mess around with the gain settings too.

Title: Re: Decware ZMF OTL Distortion
Post by Lon on 09/02/24 at 17:44:51

Like Will I suspect part of the problem to be "gain adjustment" between the preamp and the amplifier itself. I would experiment with the incoming gain, the outgoing gain, and the gain on the amp. Perhaps starting off with Steve's process for setting up the CSP3 preamps.

Title: Re: Decware ZMF OTL Distortion
Post by will on 09/02/24 at 18:37:36

Yes, I find that lowering the gain of my DAC output some (right now set at 93 in my software player Amarra) helps with "early" distortion that can develop here from multiple "gain stages" pumping up the dynamics and all progressively and together. But since you are just now experiencing this, I thought checking the gains of the new amp might be worth a look 1st, looking like it has its own a pre-stage.

Also, I don't recall having issues with my CSP3 input with 6N1Ps, I just tend to prefer other tubes there. Not sure how this might relate to your new amp, but with my Toriis, I did have problems with 6N1Ps. For the full story, both amps were some of the 1st ones made of each version, so this may have been adjusted for others. And I can't remember if it was with my Torii III, or my Torii IV (thinking maybe both), but I got what was to me, early distortion just from using 6N1Ps as inputs. Details are fuzzy, but I think it had to do with the 6N1P drawing enough more current than 6DJ8/6922s, that they caused my particular amp to fall a bit out of parameters and distort without a lot of volume.

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/02/24 at 18:50:56

Thanks for the insights. I initially tested on two different dacs. A denafrips pontus ii for my speakers and zen torii and a rme adi-2 dac for my head phone setup. I could hear distortion on both setups. I haven't messed around with the gain settings yet but plan to later today. I will keep you all posted.

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/02/24 at 20:39:12

Update: I played around with the gain settings on the RCA Input. It seems there is still distortion. The distortion is noticeable on the headphone output so i've been testing it through that. Even at the lowest gain on the input. I'll update again when new set of tubes comes in. At this point, i'm out of ideas.

Title: Re: Decware ZMF OTL Distortion
Post by Lon on 09/02/24 at 22:36:46

Well, you tried. I am at a loss for further steps. . . hope you can get some great advice and find a solution.

Title: Re: Decware ZMF OTL Distortion
Post by funch on 09/03/24 at 05:25:15

Try turning down the trimmer pot's in the back. You could be overdriving the input stage.

Title: Re: Decware ZMF OTL Distortion
Post by Lon on 09/03/24 at 08:10:30

YES! I thought you might have done that, If you have not done that it is well worth trying--that was my first suspicion as to the cause of what you were hearing.

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/03/24 at 13:51:58

I've tried trimming down the pots in the back. Even at the lowest position, the distortion is still there.

Title: Re: Decware ZMF OTL Distortion
Post by Steve Deckert on 09/03/24 at 15:55:51

The ZMF OTL was designed for 6N6P output tubes. Using 6N1P or anything else is going to compromise performance.

OTL amps are for high impedance headphones. The lower the impedance the less power output.

Steve : )

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/03/24 at 16:22:01

Hi Steve, thanks for the reply :)

I did use 6N6P output tubes. I have a new set coming just in case.

I have used this unit as a preamp for my speaker setup (Zen Torii Mk V) a few times already.

I know the main use case for the ZMF OTL is headphones but from a line stage perspective how does this unit compare to the CSP3 25th Anniv. ?

Title: Re: Decware ZMF OTL Distortion
Post by 1stwattlife on 09/04/24 at 15:46:48

>FirstWatt grabs popcorn, eagerly awaiting the answer...<

;)

Title: Re: Decware ZMF OTL Distortion
Post by ck293 on 09/10/24 at 16:35:26

Update: I figured it out. I was daisy chaining the RCA signal. DAC >> Preamp >> Subwoofer >> Speaker
I have 2 subs (KEF KF92) which have RCA Ins and Outs. For whatever the reason the ZMF OTL distorts when the signal path is configured this way.

Title: Re: Decware ZMF OTL Distortion
Post by Lon on 09/10/24 at 18:48:58

Wow. Glad you discovered that!