DIY Transistor Amp Using NVA Boards

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Cressy Snr
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#286 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Finally did something with the little Mission speakers that I have been meaning to do since I got them.
I replaced the cheapo 4.7uF electrolytic tweeter cap with a Jantzen Standard blue metallised poly cap of the same value. Another veil lifted and all that jazz. Didn’t realise the ring tweeter fitted to the mkII version (Mission 731i) was quite so fast and smooth until I fitted the Jantzen caps to the crossover. These thirty year old Missions are excellent little budget speakers.
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Niiiice.
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Cressy Snr
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#287 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Cabs might look like the usual budget fare with fancy plastic front, disguising chipboard baffle:
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But inside it’s a different story:
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Bass/mid driver is mounted on four posts connected to rear ABS plastic tray, which means that the driver is somewhat decoupled from the chipboard carcass. The tweeter is mounted on the ABS front baffle, so that it does not share any mountings with the bass driver, so decoupling it also.
I would have found difficulty in working out how to get it apart, had not an ebay seller been selling a pair of the earlier 731 cabs without drivers.

Interesting (well I think so.)
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#288 Re: DIY Transistor Amp Using NVA Boards

Post by IslandPink »

It is interesting - haven't seen anything like that before. I assume there's some sort of gasket all the way round ?
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#289 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

IslandPink wrote: Sat Jun 11, 2022 2:03 pm It is interesting - haven't seen anything like that before. I assume there's some sort of gasket all the way round ?
Yep, there’s a rubber gasket around the baffle plate edge, then another neoprene gasket to seal the woofer against the rear of the baffle.
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#290 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Got the a20 to work with the fanes, modded the p20 with a shunted tocos pot and a potential divider on the output of of to pull it right down from the 2v of a cd player to about 400mv.

It is now useable, pulls about 15w out of the wall instead of 250 ish w that the f5 pulls out of the wall so will cost less to run it.

I prefer the f5/b1, but the a20/p20 is perfectly adequete and perfectly listenable for a little amp now its had the feedback level altered.

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#291 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Just seen that you can buy a potential divider shoved in a phono plug to do the same job from rothwell audio
About 30 quid or so for a plug in solution
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#292 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Today I was mainly engaged in ‘stabilising’ the solid state amp. I fitted a Zobel network across the speaker terminals plus a parallel LR circuit in series with the output, all pretty standard stuff.

I must say that despite RD’s objections to such things I can’t detect the slightest degradation to the sound quality after doing the work. Maybe I’m cloth-eared. Anyway at least I can safely use any old speaker cable now.
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Nick
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#293 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Anyway at least I can safely use any old speaker cable now.
Not sure you can say that for certain. You would need to try and push it into instability by doing things like using a 1R load with a 1uf cap across it and looking at the square wave response, But given that the amps response to oscillating reportedly is to die, not sure that's a safe thing to do.

But what you have done is unlikely to make it less stable, and probably has made it more stable.
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#294 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Well it started because the LS2 cable I have is not long enough to reach the Mission speaker terminals. I had to buy something else that was long enough: not NVA of course.

So then the machine was not giving peace of mind and as Pirsig says: to gain peace of mind, either the machine or the user needs to change. So I changed the machine with reference to Rod Elliott’s site.

Elliott argues that no-one in their right mind builds a conventional solid state amp without at least a Zobel, even if on the surface, the amp appears to be stable. He then reckons that to make something that tolerates a wider range of speaker cable types the LR network in series with the output is desirable. So that’s what I did.

Nevertheless I wouldn’t dare put a 1R/1uF load on it. I don’t want any flash bang pyrotechnics 🙂

I mean, the amp has twin PSUs, 6dB higher feedback levels, Zobel compensation and an inductor resistor network on the output. It probably no longer qualifies as a NVA amp. I’ve messed with and buggered the whole philosophy behind it.
Last edited by Cressy Snr on Sat Jul 16, 2022 7:57 pm, edited 1 time in total.
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Nick
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#295 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Elliott gets my vote on that one.
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#296 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

I’ve been building amps now for 3 years . I’ve built 20 or so amps and wouldnt know a zobel network if you pointed to it .

Elliot comments seem unnecessary to me .
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#297 Re: DIY Transistor Amp Using NVA Boards

Post by pre65 »

Daniel Quinn wrote: Sat Jul 16, 2022 7:58 pm I’ve been building amps now for 3 years . I’ve built 20 or so amps and wouldn't know a Zobel network if you pointed to it .

Elliot comments seem unnecessary to me .
Well they would to such a rank beginner. :lol:
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Nick
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#298 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Daniel Quinn wrote: Sat Jul 16, 2022 7:58 pm I’ve been building amps now for 3 years . I’ve built 20 or so amps and wouldnt know a zobel network if you pointed to it .

Elliot comments seem unnecessary to me .
I would say the relevant statistic would be how many you had designed.

It may be that the NVA circuit doesn't need them, but given the requirements the makers place on the loudspeaker cables, it certainly seems as if the LR network wont do any harm.
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#299 Re: DIY Transistor Amp Using NVA Boards

Post by ed »

Nick wrote: Sat Jul 16, 2022 7:49 pm Elliott gets my vote on that one.
+1
My simple latfet pp amp took out my dac and squeezebox before I took Rod Elliot's advice. Still not sure how it affected stuff upstream but the zobel sure stopeed the banshee howling.
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#300 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

For the benefit of DQ. From Elliott:

3.4 - Output Stage Stability
It is a simple fact of life that an emitter follower (whether Darlington or compound) is perfectly happy to become an oscillator - generally at very high frequencies. This is especially true when the output lead looks like a tuned circuit. A length of speaker cable, while quite innocuous at audio frequencies, is a transmission line at some frequency determined by its length, conductor diameter and conductor spacing. A copy of the ARRL handbook (from any year) will provide all the formulae needed to calculate this, if you really want to go that far.

All power amplifiers (well, nearly all) use emitter follower type output stages, and when a speaker lead and speaker (or even a non-inductive dummy load) are connected, oscillation often results. This is nearly always when the amp is driven, and is more likely when current is being drawn from the circuit. It is a little sad that the compound pair is actually more prone to this errant behaviour than a Darlington, possibly because the driver is the controlling element (and its emitter is connected to the load), and has a higher bandwidth.

Some of the 'super' cables - much beloved by audiophiles - are often worse in this respect for their ability to act as RF transmission lines than ordinary Figure-8, zip cord or 3-core mains flex, and are therefore more likely to cause this problem.

Figure 10
The Standard Output Arrangement For Power Amp Stability:
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The conventional Zobel network (consisting of the 10 Ohm resistor and 100nF capacitor) generally swamps the external transmission line effect of the speaker cables and loudspeaker internal wiring, and provides stability under most normal operating conditions.

In a great many amplifiers, the amp may oscillate with no load or speaker cables attached, and a Zobel network as shown stops this, too. The reasons are a little difficult to see at first, but can be traced to small amounts of stray inductance and capacitance around the output stage in particular. At very high frequencies, these strays can easily form a tuned circuit, causing phase shift between the amp's output and inverting input. At these high frequencies, few amplifiers have a great deal of phase margin (the difference between the amplifier's phase shift and 180°). Any stray inductance and/or capacitance may only need to create a few additional degrees of phase shift to cause oscillation. Because there is very little feedback at such high frequencies, the overall impedance can be much higher than expected.

At these frequencies, the Zobel capacitor is essentially a short circuit, so there is now a 10 ohm resistor in parallel with a high impedance tuned circuit. The 10 ohm resistor ruins the Q of the tuned circuit(s), and applies heavy damping, thus negating the phase shift to a large degree and restoring stability. Personally, I don't recommend that this network be omitted from any amplifier, even if it appears to be stable without it.

With capacitive loading (as may be the case when a loudspeaker and passive crossover are connected), the Zobel network has very little additional effect - may have no effect whatsoever. The only sure way to prevent oscillation or severe ringing with highly capacitive cables is to include an inductor in the output of the amplifier. This should be bypassed with a suitable resistor to reduce the Q of the inductor, and the typical arrangement is shown in Fig 10. For readers wishing to explore this in greater depth, read 'The Audio Power Interface’ In many cases it might be better to use a far lower resistance than the 10 Ohms normally specified - I am thinking around 1 Ohm or so. Some National Semiconductor power opamps specify 2.7 ohms as the optimum. Ideally, cables with low inductance and high capacitance should always have an additional 100nF/10 ohm Zobel network at the loudspeaker end. When this is done, the cable no longer appears as a capacitor at high frequencies. Regrettably, few (if any) loudspeaker manufacturers see fit to include this at the input terminals.

Another alternative is to include a resistor in series with the output of the amplifier, but this will naturally have the dual effect of reducing power output and reducing damping factor. At resistor values sufficient to prevent oscillation, the above losses become excessive - and all wasted power must be converted into heat in the resistor.

The choice of inductor size is not difficult - for an 8 Ohm load it will be typically a maximum of 20µH, any larger than this will cause unacceptable attenuation of high frequencies. A 6µH inductor as shown in Figure 10 will introduce a low frequency loss (assuming 0.03 Ohm resistance) of 0.03dB and will be about 0.2dB down at 20kHz. These losses are insignificant, and will not be audible. In contrast, ringing (or in extreme cases, oscillation) of the output devices will be audible (even at very low levels) as increased distortion, and in extreme cases may destroy the transistors


Elliott R (2006) Power Amplifier Design Guidelines - Elliott Sound Products https://sound-au.com/amp_design.htm#s34

Fig 10 above is the arrangement I fitted to my own amp. There are no ill effects that I can hear. In fact having been listening for a couple of hours this morning, there are one or two audible advantages.
I’m using a range of music and will comment later on and cross reference what is happening to what I believe is the causing the improvements.

Note I’m saying it is what I BELIEVE. I’m not saying it proves correlation, it’s simply an informed opinion.
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