Inverted OTL with 6SN7/6AS7

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Wolfgang
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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Fri Aug 30, 2019 7:39 pm

Is the issue with the Neurochrome/XRK solution suitability down to them being unable to handle the 250V scenario or is there something else? I know the XRK board specifies that it can handle upto 48V of DC and assume the neurochrome will be similar as the designs are quite alike.
It is the -250V issue and the lowest possible trigger level. These circuits have been designed for solid state amps. OTLs and higher voltages need a little different approach if the protection should be really meaningful.

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Nick
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Re: Inverted OTL with 6SN7/6AS7

Post by Nick » Fri Aug 30, 2019 7:42 pm

The diodes on the input will stop the voltage going above or below 0.6v the supply rails, the resistor will limit the current at those times. Though I agree 250v may be problematic, depends on the power supply for the logic, a zener 1v above the normal supply voltage across the rails would provide a path to ground,
Resistance isn't futile it's V / I.

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Ray P
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Re: Inverted OTL with 6SN7/6AS7

Post by Ray P » Fri Aug 30, 2019 10:04 pm

Wolfgang wrote:
Fri Aug 30, 2019 7:33 pm
I don't know what's more important or sounds "better". Originally I wanted to use the Salas BIB (got 2 PCBs) but their max input voltage is 40V and the 30V toroids have 43V after rectification. If you use tailor made transformers it should work. But you will need big heat sinks for this !
The SLBs on the other hand have an incredible influence on the final sound and I wouldn't want to give up even 1% of that. It wouldn't matter if the mains voltage fluctuates a little. You simply won't get the max swing of 16Vp-p into 15R. But you have so much power for every dynamic situation that it doesn't matter. My output stage operates now at 39V instead of originally 42V and there is no problem.
I asked not because of a relative sound quality perspective but from that of the DC offset; I assume that, because of the asymetric voltages needed at the output stage, that any drift in the mains voltage would result in those voltages varying by different amounts and therefore changing the DC offset?
Last edited by Ray P on Fri Aug 30, 2019 10:16 pm, edited 1 time in total.

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Re: Inverted OTL with 6SN7/6AS7

Post by Ray P » Fri Aug 30, 2019 10:15 pm

Wolfgang wrote:
Thu Aug 29, 2019 4:07 pm
The InvOTL needs protection from +/- 40V and -250V. The trigger level for the latter needs to be as low as possible (without being triggered by the DC offset!) in order to give enough time for the SSRs to cut it off before it can reach high enough levels at the output that could do damage to the speakers and/or the SSRs (60V upper limit). The rising time for the 40V is not so critical.
If this is true, why do you need two trigger levels? Am I missing something because it seems to me that DC at the output is unwanted regardless of whatever is causing it so you just need a circuit that responds quickly enough to the worst case scenario, i.e one trigger level.

Then the question shifts to 'can the circuit tolerate the worst case DC voltage?'.

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Ray P
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Re: Inverted OTL with 6SN7/6AS7

Post by Ray P » Fri Aug 30, 2019 10:28 pm

Of course, there is an alternative, non-technical approach that might be woth considering, that of basic risk management. If one lists different scenarios and assess their probability (of occurring) against their impact (what would happen should they occur) you can reach sound conclusions about mitigation strategies, which may include accepting some level of risk.

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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Fri Aug 30, 2019 11:41 pm

I asked not because of a relative sound quality perspective but from that of the DC offset; I assume that, because of the asymetric voltages needed at the output stage, that any drift in the mains voltage would result in those voltages varying by different amounts and therefore changing the DC offset?
Any drift in the mains voltage would effect both rails equally. Theoretically you have a good point here. Because the driver stage has a reg. psu and the output stage wouldn't have one, using the SLB, the bias voltage which is derived from the driver stage psu would be stable while the voltage of the output stage psu would shift a little higher or lower. I don't think it matters for the DC offset because the current stays the same for both rails.Only the absolute amount of the current would change a little to higher or lower, but not relatively between the rails. The voltage drop would be also a little less or more but equal for both rails.
If this is true, why do you need two trigger levels? Am I missing something because it seems to me that DC at the output is unwanted regardless of whatever is causing it so you just need a circuit that responds quickly enough to the worst case scenario, i.e one trigger level.

Then the question shifts to 'can the circuit tolerate the worst case DC voltage?'.
Yes, one trigger level only for the worst case scenario but defined by the higher voltage so that it cannot rise high enough and damage the speaker or make a loud disturbing sound. If the circuit can handle the higher voltage it will also deal with the lower voltage. That's why we cannot use circuits that have been designed for 40V. At least I wouldn't feel comfortable to play with fire and trust that it wold work at much higher voltages.
Of course, there is an alternative, non-technical approach that might be woth considering, that of basic risk management. If one lists different scenarios and assess their probability (of occurring) against their impact (what would happen should they occur) you can reach sound conclusions about mitigation strategies, which may include accepting some level of risk.
That's how I am listening since I have built this amp. And I had one faulty tube so far. The sound it made shocked the hell out of me. :mrgreen:

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Ray P
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Re: Inverted OTL with 6SN7/6AS7

Post by Ray P » Sat Aug 31, 2019 7:02 am

Wolfgang wrote:
Fri Aug 30, 2019 11:41 pm
That's how I am listening since I have built this amp. And I had one faulty tube so far. The sound it made shocked the hell out of me. :mrgreen:
What was the DC offset that resulted from the tube failure you experience?

The XRK/Neurochrome solutions would obviously catch any upward drift of DC offset and would cope with the 40V scenario so might that mitigate the majority of DC offset scenarios and, if that is true, give you some reduction in the risk of damaging your drive units?

I don't know how likely the 250V scenaro is but I'm wondering if something is better than nothing, at least in the immediate term?

I assume that should the 250V event occur the XRK/Neurochrome board would likely be a victim too.

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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Sat Aug 31, 2019 4:54 pm

The DC offset is 100% under control and doesn't drift. Only during the warming up of the tubes and the resistors(about 10min) it slowly settles down to the once set value. One can set it to 0V with +/- 20mV.

The problem was clearly a bad tube. It was the first tube in line and from my listening position I saw a dark red "flash" like a small red cloud appearing inside the tube lasting for a split second. I interpreted it as a typical discharging from the anode to the grid and this tube tested later as "gassy". I didn't check the DC offset after this event. But I do from time to time in order to get a feeling for any changes and there are no more changes.

A very fast acting DC protection would have disconnected the speakers and the B+ (that's how I will set it up) and this discharging would have hardly made a sound. After about 10sec the amp would have played music again. In case of a physical/permanent short circuit in the tubes it would have been permanently disconnected.

I tested my protection circuit with -300V DC and - 0,8V for the neg/+1,8V for the positive side (-0,8/+1,8V are the lowest possible trigger values for this circuit) and it shuts down the moment it is connected.

Typical DC protection circuits start to trigger around 2V so it could never be a problem with any DC offset.The XRK has a much slower reaction time at lower DC than at max: "It is designed to activate at voltages above 1.5vdc in about 1 second and 48vdc in 25 milliseconds." It wouldn't work for the InvOTL.

Since all my tubes tested ok I feel safe. My circuit will be anyway ready very soon.

Sorry for the confusion with the different trigger levels but this happens sometimes to me when I am very focused on a specific problem and lose a bit the overview because of the details. Same problem with Kelvin's circuit. I couldn't see the forest for the trees.

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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Tue Sep 03, 2019 7:15 pm

That's how it looks and sounds when you connect -250V to a 8ohm Lowther driver using the DC protection circuit:

https://drive.google.com/file/d/1FXX17p ... sp=sharing

Not bad. I am satisfied with the result.

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Ray P
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Re: Inverted OTL with 6SN7/6AS7

Post by Ray P » Tue Sep 03, 2019 10:35 pm

Wolfgang wrote:
Tue Sep 03, 2019 7:15 pm
Not bad. I am satisfied with the result.
That took a brave man!

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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Wed Sep 04, 2019 5:59 pm

That took a brave man!
I tested all the functions at different voltages before I hooked it up to the DX4 for a final test under real conditions.

The reaction time was ok.
DC protection.png
Now I will find out with some listening tests whether it has any side effects on the sound.


If it all works out I will also replace the TVS diode in the Pinnacles with this circuit.

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Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang » Fri Sep 06, 2019 3:51 pm

I am closing in on my DC protection circuit for a final test in the amp.

But I have still some open questions.

I could use a single mechanical relay for disconnecting/connecting the speakers or two SSRs, one for the 250V (Vishay LH1540/350V max/20R on resistance), the second for the speakers (IXYS LCA715/60Vmax/0,15R On resistance).

There are some issues that speak against using a mechanical relay: relay contact bounce and flyback voltages. The latter could be solved with some additional circuit to some degree.

Using 2 SSRs with little different switching times (LH1540 a little faster than the LCA715 which would be good in this position/ see schematics) : How accurate are these data sheet numbers under practical conditions?
The LH1540 can be connected for DC or AC/DC. Are there any practical benefits connecting it DC?

The benefit of using SSRs: no mechanical contacts and some positive audible influence on the signal (XRK also mentions that on the etsy page and their DC protection circuit:" In fact, at higher power it actually reduces THD by a small amount (see last Figure) - an effect also observed by others implementing SSR relays." ) That's why I would prefer using SSRs. I can clearly notice a very positive (minor) influence on the sound with my active studio monitors using 4 Burmester 150W modules and with the InvOTL. So there must be something going on in this direction.
Inv OTL final v.p..png

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