DIY Amps for NX-Oticas

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poseidonsvoice

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Re: DIY Amps for NX-Oticas
« Reply #180 on: 23 Nov 2021, 12:20 pm »
Jaytor,

Nice progress on your 300B interstage amplifier. I am always pleased to see another diy'er who takes as much of an interest in proper wiring practices, grounding practices and overall neatness inside the hood. It's my firm belief that a qualified tech or the manufacturer should be impressed if they look under the hood and one should be able to 'understand' the circuit if you follow the path/traces.

I like all the upgrades you have implemented, particularly the Maida regulator (which is a staple on any of my tube builds), the ISS board (a no brainer), as well as the Tent Labs regulated filament supplies for the 300B's and Pete Millet's regulated supply for the driver stage. These are areas where most manufacturers will skimp and in my opinion, the sonics of the 300B drop down to mid-fi level.

One area I would consider experimenting is in your balanced input connection. In my experience the Jensen input transformers and Lundahl Cobalt Amorphous core are quite excellent but still lagging behind Tom Christiansen's Universal Buffer design which is basically a straight wire with gain. When you think about it, you really want the cleanest design for the small level signals prior to applying gain. Or else , that level of detail is lost forever.

The other area I would consider experimenting is the speaker protection circuit. Most I have played with including the ones from the DIYAudio store affect the sound. Moreover, they are prone to failure and arcing compared to MOSFET based designs. I would definitely consider Tom's Guardian-86 if compatible. That being said, this is the first time I have seen a speaker protection circuit in a transformer coupled tube design which is interesting. I would've figured the output transformer would provide some degree of protection should a disastrous DC mishap happen, although having a delayed startup is always a great thing which is one of the reasons I use it in cap coupled solid state designs and/or a tube design. The other thing you can try is a high voltage B+ delay and if you vary the time constant to 30 seconds to 1 minute, that should take care of any initial noise issues as the amp powers up. It’s just that many speaker protection circuits aside from Tom’s can be an impediment to the sonics since they are directly in the signal path.

The use of higher end passive parts, i.e. caps, RCA's, XLR jacks, IEC's etc....is nice icing on the cake. But like most tube designs, much of the distortion is going to be from the output transformers (especially in the bass and at high output levels) but it looks like you have invested well here. I am looking into Monolithic Magnetics myself.

Carry on! You have a $20K tube amp in the making! Very impressive!

Best,
Anand.

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #181 on: 23 Nov 2021, 04:54 pm »
Thanks for the complements and feedback.

The speaker protection circuit is really just used for delayed start as you suggested. I guess it's possible that the OPT would have a short that could cause DC, but this is pretty unlikely.

I thought about a delayed B+, but based on the reading I did, it didn't seem that necessary for this kind of amp, and I couldn't figure out an easy way to do this without adding a bunch of hardware and I didn't want to make the enclosure any larger (or more cramped). So I figured that delaying the speaker connection would be a good compromise. I realize that adding the relay does add some signal degradation, but since most of the better sounding commercial amps I've owned uses a similar circuit, I decided to go for it.

To be honest, I've never tried a FET based speaker turn-on/protection circuit. I've been pretty impressed with Tom's products (particularly for the $$), so I should give this a try in a future build.

I used Tom's Purifi input board (which I believe is the same circuit as his Universal buffer) in a couple of amps and have been happy with the performance. I also like the AMB labs A24 which is a similar design (although uses different op amps). I tend to prefer this board since I can built it myself, it's a single channel and thus smaller, and I can pick and choose the components I use.

But for this amp, I didn't want to take the room for the board and necessary power supply, and figured that most tube amps use a transformer for balanced input. From what I understand, the biggest compromise from a transformer is at the frequency extremes (particularly bass). Since I'm using separate servo subs for the lowest 2-3 octaves, I wasn't too concerned about this. But I might try an experiment at some point and build a small separate active balanced to SE converter that I can place right next to the amp to see how it compares. Of course, that will need to go through a couple extra connectors, so it's not going to be quite as good as integrating the circuit in the amp.

I think I only have a few hours of work to go before I can test out the first amp. I want to dig through my parts bin to find a high-value resistor I can use as a bleeder on the power supply so the caps don't hold a lethal charge for a long time. Do you have a recommendation for value? I was thinking 500K or so.

poseidonsvoice

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Re: DIY Amps for NX-Oticas
« Reply #182 on: 23 Nov 2021, 05:22 pm »
Jaytor,

I use 220K resistors quite a bit as bleeders. You can always parallel 2 of your 500K if need be. Remember to calculate the dissipation across that resistor using the power equation so you size the resistor appropriately. I usually go 2-3X’s that for safe operation.

Another handy tool is to build a bleeder wand. Blue Glow Electronics has a nice video on this:

https://m.youtube.com/watch?v=gLDCV-n1BDw

Best,
Anand.


mkrawcz

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Re: DIY Amps for NX-Oticas
« Reply #183 on: 23 Nov 2021, 07:00 pm »
To be honest, I've never tried a FET based speaker turn-on/protection circuit. I've been pretty impressed with Tom's products (particularly for the $$), so I should give this a try in a future build.
One thing about Tom’s Guardian circuits is they do not affect the sound quality at all. I’ve gone back and forth a lot. 

AKLegal

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Re: DIY Amps for NX-Oticas
« Reply #184 on: 23 Nov 2021, 10:36 pm »
Jury is still out. I noticed a channel imbalance after installing in my system and brought it back to the bench. When I hooked up the scope to figure out what was causing it, I noticed that I was getting a high frequency oscillation. I tracked this down to the modification I had made to the DCG3 to couple the positive and negative halves by creating a virtual ground in the feedback connection. The circuit did not like this, so I went back to separate ground connections which eliminated the oscillation. I had to order some new resistors to implement this change.

At the same time, I replaced the series resistors in the attenuator. I had asked Khozmo to use shunt resistors set up for a 5K series resistor, but I don't think they did this, and the output level was way to high. A setting of 20 out of 60 was way too loud. So I replaced the 1/2W 5K Audio Note Silver Tantulums with 2W 10K parts. Even with the 10K parts, I'm not able to use much of the range, so I suspect the attenuator was optimized for a 20K series resistor.

I had similar issues with my Khozmo ladder attenuator. The two boards I received were not the same total resistance, one was 50K the other was 10K. I ordered 10K. The main control board (the one with the microprocessors) was set to 50K. I told Khozmo that the main board was the wrong resistance but he sent me another secondary 10K board instead. So I had to remove the resistors on the main board and replace them with the ones I removed from the secondary board. Not that big of a deal it just cost me some time. I got to use my hot air solder gun so that was a plus.

Now that everything is fixed the sound is amazing. Khozmo makes superior gear and depending on how much gain I set my output buffers at, I can use most of the 60 steps - I get up to 40 before its too loud. I just need to order a chassis. I've got the Khozmo ladder attenuator powered by a AMB sigma 11. The attenuator feeds into two Neurochrome universal buffers, one per channel, each fed by their own Sigma 22. Yes, the power supply is overkill and will be in a separate chassis.

I love the look of the cases for your preamp and tube amps. They remind me of the front faces of Stello audio gear from a few years ago- a sort of soft, sandblasted silver.



 

   

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #185 on: 23 Nov 2021, 11:03 pm »
I've got the Khozmo ladder attenuator powered by a AMB sigma 11. The attenuator feeds into two Neurochrome universal buffers, one per channel, each fed by their own Sigma 22. Yes, the power supply is overkill and will be in a separate chassis.

That's pretty similar to my first preamp (see post #17 in this thread), although I'm only using a single Sigma 22 for both channels. I'm using the AMB A24 boards instead of Tom's boards, but the topology is very similar. This preamp still sounds excellent as I imagine yours does.

I love the look of the cases for your preamp and tube amps. They remind me of the front faces of Stello audio gear from a few years ago- a sort of soft, sandblasted silver.

Thanks. I like the powder coated finish that Front Panel Express provides, as well as their flexibility and design tool, although their prices are pretty steep, particularly compared to what you can get from ModuShop. But I don't do these projects to save money and decided It's worth it to me for the increased satisfaction with the results.

mkrawcz

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Re: DIY Amps for NX-Oticas
« Reply #186 on: 23 Nov 2021, 11:52 pm »
The attenuator feeds into two Neurochrome universal buffers, one per channel, each fed by their own Sigma 22. Yes, the power supply is overkill and will be in a separate chassis.
 
I just converted my Folsom 7293 to balanced using a Neurochrome Universal buffer and the results are great. Not only did it take care of some pretty awful grounding issues, it widened and deepened the soundstage of the amp to fall more in line with the Neurochrome amps. I also added the Guardian 86 modules which eliminated the pops and thuds associated with turning the amp on and off.

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #187 on: 24 Nov 2021, 12:07 am »
I remember one of the reasons I didn't consider the Guardian is that I didn't want to leave the OPT unloaded during the turn-on delay. With a relay, I could switch a 25W 8ohm resistor in place of the speakers. I've read that leaving the transformer output disconnected on a SET amp can damage the tubes.