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Old 11th July 2011, 09:07 PM   #11
farjon is offline farjon  Brazil
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Quote:
Originally Posted by ostripper View Post
The safety resistors can be used to measure OP current + the current demands of the input circuitry. To measure the output Re's directly is more accurate and you can see if any OP devices are failing to "share" (gross Hfe mismatches). Most ON semi output devices from the same order will typically read within a mV across one of the .22R resistors. For example , these amps are typically set at 10-12mV.
So, let's TP on every Re...
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Old 12th July 2011, 04:28 AM   #12
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Default Just a few ideas/options.

Quote:
Originally Posted by Sound_Buster View Post
So, let's TP on every Re...
Most builders will run through their pairs the first time . After that, one would just measure 22mv across the TP's , run the amp , maybe readjust. The emitter resistor leads are 20mm away from any rail.

A few thoughts to ponder .... I have built this particular amp with cascoded input stages , one pair with (cascoded)SS9014 low noise BJT's and one pair with
cascoded 2n4416 Jfets. The BJT's were quieter (only measurable with a CRO) , the Jfet's subjectively sounded better (I am aware of the psychology).In Either version, the differences were minimal.

Benefit's of the cascode are the wider choice of input devices , less thermal drift as the input pair Vce is lower =cooler. Of course you need a minimum of 2 resistors or resistor/zener to bias the cascode , increasing the BOM. The Jfet's will have an effect on the loop gain of the amp , reducing the effectiveness of TMC. With this amp you can choose standard miller compensation , so a FET input pair amp is still possible. The "tradeoff" is more distortion vs. A "slightly different" sound. I have no AP , but in simulation , a slight shift in harmonic content can be seen with the Jfets.

I would appreciate opinions as to whether I should include the cascode.

The layout features would be :
1. Jfet or BJT - I know both work and have the values already worked out.

2. Total "opt out" - jumper the cascode , use the 2 high Vce input pair (992/1845).

3. Cascode voltage reference can be just a simple resistive divider or zener referenced (or omitted).

I would like feedback as to whether this would be a good addition or would it overly complicate the design. It would be good if this design could satisfy both the "new" builder and someone with higher expectations (picky people ).

It was mentioned to me that the power supply capacity might be a bit low. Is up to 22,000uf X 2 (panasonic) ECE-T1JP223FA Panasonic Electronic Components Aluminum Electrolytic Capacitors - Snap In Insufficient for a 150W amp ?

I think this would be quite sufficient The class A design usually has much poorer PSRR and lower or no negative feedback , thus needing 100Kuf + CLC/CRC to have acceptable performance. An AB amp (this one) is 70+ db PSRR "right out of the box" . Our popular goldmund clone only uses 2 Apex jr. 8200uf's , the AKSA's use 4 X 10K onboard (good deal). All these far exceed any OEM I've repaired - most will get away with 10K X 2 for a STEREO 120W setup ..... pure economics.

Below 1 / JFET - TMC is really not effective (68pF could be used w/ standard miller comp.). Distortion is still 10-12 PPM @ 10K , H2 is more dominant. Whether this can be heard might depend on the "golden ear factor"

OS
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Old 12th July 2011, 08:04 AM   #13
Variac is offline Variac  United States
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I like the idea of some options, if it were clearly spelled out how to implement them or not. I found notes on the board itself to be the least threatening. Like a note saying something like:

For no cascode eliminate all asterisked components & jumper "A" to "B"

Of course room is needed to write this, but it came be done more succinctly than my example..

Hoever I'm no expert on this type of amp, so you guys should decide this..

If OS could write up a short article after the board is designed in which all the options are explained clearly, it would always be available to builders.. Or diyAudio could publish it..
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Old 12th July 2011, 11:16 AM   #14
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I think the cascode is a good idea, it gives people more room to play around if they want, especially if the BJT and Jfets share the same pin outs too. This also gives greater flexibility on the input pair choices, easing the selection of a high beta pair as now the Vce restraints have been lifted.

I'd say that any economical advantage when removing the cascode + biasing would be largely irrelevant as the additional cost would be peanuts when compared to the rest of the expensive items. As Variac says, having the option to 'jumper this', if you want X to happen etc is a nice idea, it does add complexity to the overall PCB design and increases the BOM again, but if anything DIYers prefer to have options.

I think having an amplifier where you can try out the various compensation methods + different styles of input stage would be quite interesting and would probably pique quite a few peoples interests.

I also think that having three output pairs is a good choice too as it allows the amplifier to deliver a decent amount of current when set at the 150 watt @ 8 ohm target.

Another thing perhaps is that the cascode would give more flexibility on the power rails. With three output pairs, if you had a benign load to drive, you could easily up the rail voltages to give more into 8 ohms. Of course if you've got a high bias input stage this could push the input transistors beyond their comfortable limits, some small signal stuff will only tolerate 300mW and that's at 25 degrees C. 50 volt rails and a tail current of say 2mA would push them to their limits. If someone wanted to up the rails to 60 volts they'd be in danger territory.
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Old 12th July 2011, 11:45 AM   #15
dadod is offline dadod  Croatia
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Quote:
Originally Posted by ostripper View Post
Most builders will run through their pairs the first time . After that, one would just measure 22mv across the TP's , run the amp , maybe readjust. The emitter resistor leads are 20mm away from any rail.

A few thoughts to ponder .... I have built this particular amp with cascoded input stages , one pair with (cascoded)SS9014 low noise BJT's and one pair with
cascoded 2n4416 Jfets. The BJT's were quieter (only measurable with a CRO) , the Jfet's subjectively sounded better (I am aware of the psychology).In Either version, the differences were minimal.

Benefit's of the cascode are the wider choice of input devices , less thermal drift as the input pair Vce is lower =cooler. Of course you need a minimum of 2 resistors or resistor/zener to bias the cascode , increasing the BOM. The Jfet's will have an effect on the loop gain of the amp , reducing the effectiveness of TMC. With this amp you can choose standard miller compensation , so a FET input pair amp is still possible. The "tradeoff" is more distortion vs. A "slightly different" sound. I have no AP , but in simulation , a slight shift in harmonic content can be seen with the Jfets.

I would appreciate opinions as to whether I should include the cascode.

The layout features would be :
1. Jfet or BJT - I know both work and have the values already worked out.

2. Total "opt out" - jumper the cascode , use the 2 high Vce input pair (992/1845).

3. Cascode voltage reference can be just a simple resistive divider or zener referenced (or omitted).

I would like feedback as to whether this would be a good addition or would it overly complicate the design. It would be good if this design could satisfy both the "new" builder and someone with higher expectations (picky people ).

It was mentioned to me that the power supply capacity might be a bit low. Is up to 22,000uf X 2 (panasonic) ECE-T1JP223FA Panasonic Electronic Components Aluminum Electrolytic Capacitors - Snap In Insufficient for a 150W amp ?

I think this would be quite sufficient The class A design usually has much poorer PSRR and lower or no negative feedback , thus needing 100Kuf + CLC/CRC to have acceptable performance. An AB amp (this one) is 70+ db PSRR "right out of the box" . Our popular goldmund clone only uses 2 Apex jr. 8200uf's , the AKSA's use 4 X 10K onboard (good deal). All these far exceed any OEM I've repaired - most will get away with 10K X 2 for a STEREO 120W setup ..... pure economics.

Below 1 / JFET - TMC is really not effective (68pF could be used w/ standard miller comp.). Distortion is still 10-12 PPM @ 10K , H2 is more dominant. Whether this can be heard might depend on the "golden ear factor"

OS
I suggest this cascoded LTP. Cascode is floating with LTP emitters.
dado
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Old 12th July 2011, 11:51 AM   #16
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5'th .... You know , I am going to allow for boosted/separate rail capability. Standard on all my amps. In this case, even know it will be added/documented/labled , you would just leave the last 2 resistors out of the last R/C filter for the front end and hook your boosted rails there.

In general , you can use higher global rails , too. My Nikko version of this amp uses nearly 70V with a current limited trafo and 2 pair outputs. It's headroom at 8R is impressive (quite scary , even).

Variac , even as the physical is quite important , the documentation is equally important. The goal is to not scare the new constructor away or shock the class A crowd (these amps for bi-amp bass , maybe ?) , but allow for refinement if wanted - broad appeal. My house amps are already like this , 1 pair is cascoded Jfet with no TMC (just one miller cap/1 jumper/omitted resistor) , the other (2 miller caps/resistor/no jfets).

I'm not throwing unproven design's out to the crowd , BTW. I specifically chose this one for the quality/simplicity/support aspects. Also , I have abused these to the max - they keep on truckin'.

OS
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Old 12th July 2011, 12:03 PM   #17
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Quote:
Originally Posted by dadod View Post
I suggest this cascoded LTP. Cascode is floating with LTP emitters.
dado
Yes , that actually is the way I have my jfet one , but with just 2 resistor's and a small bypass cap.

If you do a poll of the OEM cascoded amps , you will see them divided into 3 camps :

1. floating emitter referenced with bypass between cascode and LTP emitters.

2. Rail to ground with bypass between cascode/ground (some use a 12v zener).

3. rail to rail floating - bypass to ground (rare).

Comments on best choice welcome (I've used 1/2 sucessfully).

PS - KISS = keep it simple !!! (just the 2 semi's + 2-4 passives)
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Last edited by ostripper; 12th July 2011 at 12:15 PM.
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Old 12th July 2011, 12:04 PM   #18
AndrewT is offline AndrewT  Scotland
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I like the cascode idea.
It adds a tiny bit of board area.
It can be omitted using zero cost jumpers.
The jFETs in the LTP can be replaced with BJTs if desired.

I love the test points idea. I'd make them mandatory.

Fused rails with 100r bypasses permanently in place.
Once the circuit is proved the empty fuseholders get 10r or 1r resistors soldered inside old fuses. This allows quiescent current measurement with very little voltage loss. Output bias current and driver bias current are measured across the test points fitted to the emitter resistors.
Finally the intended fuses replace the 1r0 resistors.

I hate the EF2. Why not EF3. Again easy to link out the pre-driver stage, if desired.
All capacitors must have at least dual pin pitch options.
All drivers and output to have provision for base stoppers.
Power input via a twisted triplet to three, very closely spaced, power terminals.
All or most signal transistors to have 4 pin pads to allow the different pin outs. bc, 2n, 2sc etc.
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Old 12th July 2011, 12:45 PM   #19
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Quote:
I hate the EF2. Why not EF3.
You "hit the nail on the head". I hate the standard EF3 , harder to thermally and electrically stabilize with modern High Ft semi's.The Cordell DBT triple is quite superior , but too complicated. The EF2 can be good if the VAS Ic is high and the driver/output beta's are high. 3 OP pairs also work very well with an EF2.

The "tradeoffs" here are:
-Larger onboard capacitance vs. design sophistication.
-Project had to be ported to an existing Class A chassis with preset parameters.
-A real world assessment of what projects members are actually building , and what skill level these projects are at.
-Just how "bulletproof" to make the project. I really feel the protection should be offboard , PIC or ta7317 based - with PMA's HEXFET ultrafast SS relay. Onboard VI sucks , except for a "road amp" , relays don't always work - unless you get the 100A/ 20$ platinum edition.


The Leach amp was good for first time constructors as it used slow devices .My present triple would not tolerate the stray inductances (wires) that you see on the typical leach build. I do know how to mitigate this , but with extra local bypass circuitry , L/R coupling. In my opinion , the leach is too hard for the first time constructor. For every leach build , 10 buy an ebay kit and 10 build a DX.

After I perfect the luxman/DBT (finish any R/D , give it the torture test) , it might be "round 2" as a boardstore kit.

OS
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Last edited by ostripper; 12th July 2011 at 12:56 PM.
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Old 12th July 2011, 01:10 PM   #20
dadod is offline dadod  Croatia
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Quote:
Originally Posted by ostripper View Post
Yes , that actually is the way I have my jfet one , but with just 2 resistor's and a small bypass cap.

If you do a poll of the OEM cascoded amps , you will see them divided into 3 camps :

1. floating emitter referenced with bypass between cascode and LTP emitters.

2. Rail to ground with bypass between cascode/ground (some use a 12v zener).

3. rail to rail floating - bypass to ground (rare).

Comments on best choice welcome (I've used 1/2 sucessfully).

PS - KISS = keep it simple !!! (just the 2 semi's + 2-4 passives)
OS
The cascode I suggested is actually proposed to me by Cordell in his book thread.

"I normally recommend caution in regard to the type of circuit you are using to bootstrap the input cascodes; I'm not crazy about injecting extra current into the LTP tail circuit. I might have used an emitter follower into whose emitter I would have flowed that current. I prefer the driven cascode that is driven by a replica of the feedback signal, which, under normal conditions, is pretty much the same as the common mode signal."

dado
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