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Old 25th May 2002, 10:58 AM   #1
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Default Current loading Opamps

The headphone people are quite right. Even in buffered applications, current loading a filter/buffer improves the sonics. I have just tried this on an Assemblage 3.1 signature, further modified. With AD825 output buffered by BUF634, I have 1 channel with buffered CRD at 1 mA, and resistor loading on another at about 1.2mA. They sound different and a lot less dry in the mid/hi than the unloaded units; definitely better. The CRD side sounds somewhat less forward than the resistor loaded side, and the effect is reversed when channel is reversed, validating my hearing.

My conclusion:- well worth shifting the operating point of even 'good' sounding opamps. The OPA 627, with its many supporters but not me, can be made to sound decent with about 1.6-2 mA loading, but still not fully open.

Does anyone have access or know source of an extender which allows me to fit a two fet current regulators on top of a DIL setup please. This will facilitate comparisons and tweaking???
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Old 25th May 2002, 12:20 PM   #2
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Default cascode current source opamp adapter

Maybe I'll design one. The BrownDog opamp adapter seems to be doing fairly well, so I am feeling inspired. If I were to do this, I would lean towards a adapter that is basically a passthrough. It would make the IC taller, but not extend beyond the 400x400 mil boundary of the DIP socket. The opamp plugs into the cascode adapter which plugs into the socket on the board. The FETs lurk under the opamp on the adapter.

I want to do some experiments with a CRD/FET cascode instead of a dual FET cascode. It would take less space and might be better albeit more expensive than 2 FETs due to the tighter current regulation of the CRD coupled with the lower capacitance of the cascoded FET.

I am not sure if I can talk BrownDog into making this one, too. It is a bit more esoteric and would involve onboard components which is sort of of out of their scope, but I'll look into it.

It might be possible to get BrownDog or someone else to make a total solution module a la LC Audio, complete with opamps and cascode current source. The drawback is you are stuck with whatever opamp is chosen, so I am not as keen on this approach. I'll follow up after Memorial Day weekend.
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Old 25th May 2002, 03:02 PM   #3
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Default Re: cascode current source opamp adapter

[QUOTE]Originally posted by morsel
[B]Maybe I'll design one. The BrownDog opamp adapter seems to be doing fairly well, so I am feeling inspired. If I were to do this, I would lean towards a adapter that is basically a passthrough. It would make the IC taller, but not extend beyond the 400x400 mil boundary of the DIP socket. The opamp plugs into the cascode adapter which plugs into the socket on the board. The FETs lurk under the opamp on the adapter.

------------------------------------------------------------------

Great one, pl get working. Two points:

1. The opamp should be exposed to free air for ventilation, and

2. Cascade thru' a second adaptor adds two many contacts. A new second tier that can be soldered onto the two leads of the existing one will be easy and cheap. All we need are a few tracks on this and two pins or flexiblelish leads.

What is the advantage of a CRD/FET? Why not just go for a dual fet to save space and work?
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Old 25th May 2002, 10:46 PM   #4
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Default Re: Re: cascode current source opamp adapter

If you really believe the extra socket contacts are going to ruin your circuit then air wire it yourself. Personally, I think that is nonsense. I got a lot of (groundless? muahaha!) complaints for not including bypass capacitors and a ground tether on the BrownDog opamp adapter design, yet the adapter works great, fits where no LC Audio module can, and accepts your choice of opamp. It is much better off as it is, and so will the cascode adapter be if I do it. Most people don't want to do microsurgery and air wiring. If it plugs in and it works it will be popular. Ironically, I will probably be able to fit a bypass cap on the cascode adapter, but it will go from +V to -V. Better than nothing. You could always balance the cascode adapter on top of your BrownDog adapter and solder it that way.

Quote:
What is the advantage of a CRD/FET? Why not just go for a dual fet to save space and work?
Take a second glance at my previous post. I'm not sure that it is better, I want to look into it. A dual FET cascode takes more space, actually. CRDs are tiny glass packages the size of small signal diodes.

If I do the cascode adapter I may use all surface mount components if I can find someone to make them. I'm off to the local surplus shop to see if I can find some FETs and CRDs to play with.
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Old 25th May 2002, 11:29 PM   #5
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Default Re: Re: Re: cascode current source opamp adapter

[QUOTEIf you really believe the extra socket contacts are going to ruin your circuit then air wire it yourself. Personally, I think that is nonsense. IIf I do the cascode adapter I may use all surface mount components if I can find someone to make them. I'm off to the local surplus shop to see if I can find some FETs and CRDs to play with. [/QUOTE]
----------------------------------------------------------

Why have more contacts than one needs to. They DO deteriorate with time.

2N crds are no longer available in UK; only J50x and CRx which are larger.
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Old 26th May 2002, 02:43 AM   #6
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Sorry to hear about the lack of small CRDs in the UK. I struck out at the local surplus shop, so I will have to wait for mail order.
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Old 27th May 2002, 03:22 AM   #7
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While waiting for FETs, etc. I decided to start looking at possible designs for a FET cascode current source adapter.

First, silly as it may sound, I evaluated the feasibility of using discrete non SMT components. 4 TO-92 FETS will barely fit between the rows of socket pins. If capacitance buffering resistors were to be included they would somehow have to be stashed on the underside. With all the holes needed for the FET leads, it would be hard to find room for the resistor mounting holes. Leaving out the resistors means the opamp will see more of the capacitance of the FETs, but it would still work, and no SMT would be involved.

Onward to the more sensible SMT option. One current source goes on each side of the adapter. Assuming 2N5484 FETS, Idss is 1-5mA with 4 being typical. Desired current is about 1mA, but without picking FETs will typically be much higher. This means that the 100 Ohm capacitance buffering resistors will need to dissipate more power than is possible for an 0805 SMT resistor, which is the largest that will comfortably fit on the board. Thus we will also need a current limiting resistor on the source of Q1, or find some SMT CRDs to use instead. Crystalonics makes such a device, but I do not know how available or economical they would be. It will be tight squeezing two resistors and two FETs on each side. The resistors would have to be 0603 to fit.
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Old 27th May 2002, 05:41 AM   #8
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Here is a rough draft using a current limiting resistor on the source of Q1 as per the Siliconix paper figure 2 with a capacitance buffering resistor between the cascode and the opamp:

http://www.vishay.com/document/70596/70596.pdf

I find myself wondering if the circuit would be better with the gate of Q2 tied to -V instead of Q1 source. Any thoughts? I have no FETs to play with atm.
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Old 27th May 2002, 09:36 AM   #9
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The drain-gate voltage will be too low, and performance will suffer.

Jocko
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Old 28th May 2002, 01:46 AM   #10
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Dyslexics untie! I think I scrambled some numbers yesterday, in fact there is no power problem with 100 Ohm resistors, the power would be 100-1000 microWatts, not milliWatts. Nevertheless, it would be nice to reduce the current to somewhere between .5 and 1mA. It would also be nice if the resistors were the same value, so there is no issue with mixing them up during assembly.
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