New Follower Amp project

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Ray P
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#1 New Follower Amp project

Post by Ray P »

I just can't stop myself collecting projects!

Now I've almost completed the 2P29L pre-amp to drive the MoFo I thought I would try a different follower power buffer, this time using some Tokin 2SK180 or THF-51S SITs

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Ray P
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#2 Re: New Follower Amp project

Post by Ray P »

I plan to use it in the same way as the MoFo, to drive the full-range units in the SLOB speaker project so want to roll-off the bottom end in the same way.

One of the DIY Audio members very kindly ran a Spice simulation for me;

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so this is pretty much my target schematic.
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Ray P
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#3 Re: New Follower Amp project

Post by Ray P »

One change I plan to make is to reduce the inductance of the choke, possible because of rolling off the bottom end. Here's a simulation of the FR of the Hammond 193V inductor (150mH) versus a Lundahl LL1694 (40mH).

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I've order a pair of the Lundahls. I'm assuming the quality of the Lundahls will be better than the Hammonds too.
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#4 Re: New Follower Amp project

Post by Ray P »

Running the SITs at around 35V and 2.5A quiescent current creates heat - I picked up a heatsink locally for about £30 - I think it's from an invertor as the chap is in the renewable energy business. It's a big piece of metal, about 500mm tall, 330mm wide and 60mm deep.

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I need to find someone to anodise it black.

I think this will be just about sufficient to manage the heat with a pair of SITs but the chap thinks he has another one stashed away so if that turns up it'll make things easier both heat wise and to assemble some sort of chassis.

The rectangular cutout gives a nice option to install in insert panel for the connectors etc.
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#5 Re: New Follower Amp project

Post by ed »

Hi Ray

any chance of a copy of your 2sk180 model?
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Ray P
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#6 Re: New Follower Amp project

Post by Ray P »

ed wrote: Sun May 23, 2021 11:00 am Hi Ray

any chance of a copy of your 2sk180 model?
Sorry Ed but they're not my simulations so I don't have a 2SK180 model - I don't even have Spice (a fool with a tool...)

The simulations were done by a chap over on DIY Audio, I'm sure if you post a request over there someone will help.
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#7 Re: New Follower Amp project

Post by Ray P »

Here's the circuit I've settled on building with the changes to the original at the top of this thread shown in red:
2SK180 Follower.png
2SK180 Follower.png (27.31 KiB) Viewed 5546 times
The aim is to roll-off the bottom end at around 150-180Hz as it will be used to drive a 'full-range' speaker in a Pass open baffle. As mentioned above the input cap has been changed to a value that will roll-off the bass at the required point but I've also changed the output cap - 180uF and an assumed 8ohm speaker load gives a LP filter with a -3dB point at about 114Hz - which gives the option to use a plastic film cap.

Slowly collecting bits and pieces for the build. It doesn't look as though the guy will come through with another heatsink though so need to get it up and rnning with a SMPS to see how the heat management works out with just the one.
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Ray P
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#8 Re: New Follower Amp project

Post by Ray P »

One of the DIY Audio members kindly ran a sim on the above schematic.

THF-51S 100nF LLundahl Sim.png
THF-51S 100nF LLundahl Sim.png (108.74 KiB) Viewed 5518 times
THF-51S 100nF Llundahl Sim freq resp.png
THF-51S 100nF Llundahl Sim freq resp.png (67.34 KiB) Viewed 5518 times
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Ray P
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#9 Re: New Follower Amp project

Post by Ray P »

I've been slowly working on mt Follower project. It's essentially similar to a MoFo but uses Tokin 2SK180 SIT devices instead of the usual Mosfets (The Tokin THF-51S I was going to use are now going in my other SIT project).

I have decided to use some Cap Mx supplies, that were originally destined for a MoFo build, so I installed and checked out the first one to see if it contained any magic smoke. Load resistor is pulling less than 100mA but it checked out fine. I'm measuring 42VDC on the output but that'll obviously drop under a proper load - target is around 38-39V for the optimal SIT operating point.

I will use this install to start learning how to use my recently purchased oscilloscope.

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The transformer sits snugly into the U channel section that forms the bottom of the chassis so all AC will be confined to the lower back part of the chassis. Phil, note all the neatly drilled and tapped holes with studs inserted to bolt the chassis together!

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Last edited by Ray P on Wed Nov 10, 2021 4:06 pm, edited 1 time in total.
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#10 Re: New Follower Amp project

Post by Ray P »

I then installed the top of the chassis ready for preparing to mate up the rest of the chassis. I think it should look rather nice with the red anodising and a nice hardwood front panel.

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Heatsinks are recycled from solar panel invertors, purchased on ebay for £20ea delivered.
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#11 Re: New Follower Amp project

Post by Ray P »

The SITs, PCBs and chokes mount on an internal U channel.

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#12 Re: New Follower Amp project

Post by Ant »

Very cool ray, that red anodising looks great
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Ray P
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#13 Re: New Follower Amp project

Post by Ray P »

Asembled and tested the second Cap MX and soft start boards today - all good.

Postie is very late today but not at all bad as she delivered a big wirewound test resistor so tomoorow I can check out the Cap MXs with a decent load, up around 1.7A
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#14 Re: New Follower Amp project

Post by Ray P »

A bit more progress today, coming together nicely...

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#15 Re: New Follower Amp project

Post by Ray P »

Back on this project today, did a little rework, moving the chokes to the rear panel to keep the airflow clear for the SITs.

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The little power supply is for biasing the SITs.
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