SLOB project

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Ray P
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#211 Re: SLOB project

Post by Ray P »

I'm sure I'm over-simplifying this so a hand if you please...

I've used an on-line filter calculator for a 180Hz crossover point for a 15ohm Lowther which gives the following output;

Capture.JPG

Assuming I'll use the 6C33C OTL amp that I've built based on Wolfgang's schematic;

Capture1.JPG

Is it as simple as substituting the 34uF cap from the calculator for the 1000uF output cap in the schematic?
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Nick
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#212 Re: SLOB project

Post by Nick »

No because it's inside the feedback loop so will alter things.
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#213 Re: SLOB project

Post by Ray P »

Cheers Nick - so I was right that I over-simplified. So should I just move the output cap outside of the feedback loop or keep the amp 'as is' and add the 34uF cap to the Lowther terminals?
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#214 Re: SLOB project

Post by Nick »

Yes, or you could add the filter on the front of the amp, calculate the -3dB point with the input resistor and a cap before that. That should let you use a smaller cap.
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#215 Re: SLOB project

Post by Ray P »

Is that 'Yes' to moving the output cap outside of the feedback loop or 'Yes' to keeping the output cap and adding the filter cap to the Lowthers, or perhaps either.

I did think about adding a filter cap on the input but was targeting the outout because it would enable an electrolytic to be replaced with a film cap with the additional benefit of a higher voltage rating to reduce the risk of B+ voltage getting to a drive unit in the event of an output valve failure.
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#216 Re: SLOB project

Post by Nick »

If you moved the feedback before the output cap then you would have the DC on the output injected back into the first stage, so I would leave that as it is. If you are worried about voltage, use a higher voltage electrolytic. But yes, add it to the output, which as far as I see would be the same as adding it to the lowther, its just the other end of a wire unless I am missing something.
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#217 Re: SLOB project

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Nick wrote: Wed Dec 02, 2020 10:24 am ...which as far as I see would be the same as adding it to the lowther, its just the other end of a wire unless I am missing something.
That's about the size of it Nick, thanks for the help. So, leave the amp alone and pop the cap on the Lowther units.

Regarding the voltage rating of the electrolytic output caps, the problem is that they get very big and expensive, however, the risk to the drive units will be mitigated with a suitable voltage rating for the roll-off cap.

It would be pretty simple to try a filter cap on the amp input too so may give that a whirl at some point.
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#218 Re: SLOB project

Post by steve s »

Years ago when I started using my lowther with a bass driver I was concerned about a cap in the signal path, I spent a while with it in and out of the circuit, quality wise I could not tell much difference.
I actually thought the cap slightly improved the lowther.
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#219 Re: SLOB project

Post by Ray P »

Thanks Steve - interesting.
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#220 Re: SLOB project

Post by chris661 »

IMO, reducing the LF load on a full-range driver is always a good thing. Reduced distortion etc etc. Bonus if you do it at the amp input - you gain headroom at the amplifier too.

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#221 Re: SLOB project

Post by Ray P »

chris661 wrote: Thu Dec 03, 2020 10:41 pm Bonus if you do it at the amp input - you gain headroom at the amplifier too.
Yes, a decent quality cap of 8.2nF on the input will give a hi-pass filter with a -3dB point at around 190Hz - should give the amps and FR drivers easier lives.

I can play around with DSP on the bass units so I have the option of a little boost if needed and I can swap in something like the Nelson Pass active crossover if the fancy takes me.
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#222 Re: SLOB project

Post by Ray P »

Or, if I decide to do away with some of the flexibility of the amp I could use an 18nF(ish) cap in place of the 1uF between the 6SN7 and 6C33C valves. As I have my 13E1 amp with a similar capability, dedicating the 6C33C amp in this way isn't a big sacrifice.
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#223 Re: SLOB project

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Ray P wrote: Fri Dec 04, 2020 7:44 pm Or, if I decide to do away with some of the flexibility of the amp I could use an 18nF(ish) cap in place of the 1uF between the 6SN7 and 6C33C valves. As I have my 13E1 amp with a similar capability, dedicating the 6C33C amp in this way isn't a big sacrifice.
Referencing this to the amp schematic at the top of this page, can someone kindly confirm that I have this correct.

The filter calculator I used shows a -3db point at 176Hz if I use an 18nF cap in place of the 1uF cap between the 6SN7 and 6C33C.

I have a pair of nice 2% 18nF silver mica caps in the spares box and I think I'll swap them in when I install the current sinks to finish the 6C33C build.

It also means that I can swap in a film cap, say with a -3dB point at 150Hz, for the electrolytic output cap.

Ta muchly
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#224 Re: SLOB project

Post by Nick »

Simply put, I don't know, as its inside the feedback loop.
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#225 Re: SLOB project

Post by ed »

Here's the cheapskate's approach. I've adjusted the dims for some unused drivers I've got on the shelf. I'm off to B&Q this afternoon to see if they'll cut me a sheet of carcass ply. It's a gamble, but if it works I've still got about 20+ sq mtrs of light ebony veneer left over from 2012 when I closed the workshop. I just checked and I've got 6 tpa3116 amps on the shelf. Not sure if I've got enough opamps for two 2 way L/R filters...but thats academic.
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