Second attempt at Building my own amp from scratch

[djk]

Something is saturating and it is sticking to the rail a bit.

 

[AndrewT]

Q21 should be protected from blowing up. Add in a collector resistor. 2N5551 does not make a good VAS transistor. Aim for Cob <5pF and preferably <3pF @ 10Vce.
Q3 must be protected from blowing up if you add your output current limiter.

 

[Vostro]

Thanks, Ill try another transistor (not sure which one yet, maybe another 2sc3788, to test) and add that resistor.
I added D3 to help Q3 during clip/saturating, I guess I should just have added another transistor to protect Q3 instead.

Ill report back after changes.

Just a thought, since sticking is at Positive halfcycle, is this causing
saturation of Q3,21 or on the Q12 side ?


Regards Vostro

 

[Vostro]

After playing with changing Q3 with no improvement, I looked elsewhere.

I was using a KSE350 for Q12, since its all all plastic body and easy to mount to heatsink.
I replaced it with a OnSemi MJE350 and sticking was still there but with max output moving from 107Vp-p to 110Vp-p, so I left it in.

This morning I removed the RC network C2/R17, sticking was completly gone but with instability, oscillation appearing as ringing, even at low levels.

I then replaced C2 with a 22pf and instability wasnt visible and sticking was very minor,
I felt uneasy about the oscillation and replaced C2/R17 with just one 100pf.
Sticking is more than with 22pf and I hope the amp will remain stable.

Attached it screenshot of clipping with 100pf in circuit.

Regards.

 

[Vostro]

This board is using a tripple EF, stability seems ok,
Will it perform and be more stable if I convert it to a double EF ?

Thanks Vostro

 

[Vostro]

Since my previous amp is working fine now, I want to make some changes.
Here is the next revision of the amp, still pretty standard design.

Again thanks for all the help so far,

I'd like to know about R17 that was suggested,
- I'm not sure about calculating is optimal value and power rating,
- at first glance I thought a nominal value of 470R to 2k should work,

I'd like to know what low power and med power transistors I could use for the VAS stage,
- If tried 2SC3953 and 2SC3788 but I can't source reputable pieces of these here.
- I also buy for RS but they don't seen to stock them,
- I've used MJE340 and they worked for me, but heard they not optimal.

I'm going to use a mix of through hole and surface mount parts (single sided pcb board, since I'm making them toner transfer)

I've not decided if I'll make the pcb with the VI limiter, or if I'll leave it optional on small breakout board.

Regards

 

[AndrewT]

forget mje340 for VAS duty.
You need high fT, low Cob, highish hFE and fairly linear hFE with change in current.
Same applies to Q12 since it has to react to the change coming from the VAS.

 

[Vostro]

Thanks AndrewT your advice is always appreciated,

Now I just need some suggestions for The Low Power and High Power VAS transistors.
Through hole and surface mount, preferably available from RS Components.

Thanks

 

[jaycee]

Unfortunately such devices are getting hard to find... the original purpose of high speed, high voltage, medium current and low capacitance transistors was in CRT applications.

Sanyo used to have some excellent devices, but since being bought by Onsemi, most of them seem to have been discontinued.

Fairchild have their 2SA1381/2SC3503 clones which are good, and you may be able to get them direct from Fairchild as samples.

 

[AndrewT]

Greg did a fantastic set of look up tables to allow you to select suitable devices.
Use it.
The resource should not be wasted.

 

[jaycee]

VAS Transistors

The problem is getting most of those. The 2SA1209/2SC2911 used to be easy to get - but since the Onsemi take over, cant find anyone selling them except Profusion - not really suitable for hobbyists.

The 2SC3421, 2SC3423 and complements are now discontinued by Toshiba.
Likewise, the Sanyo devices have become unobtainium.

 

[jaycee]

Diodes Inc/Zetex DZT5401/DZT5551 seem interesting. They are in a SOT-223 package and rated for 1W.

Also ZXTN5551G/ZXTN5551G similar...

 

[Vostro]

I used 2SC3953 (from RS) for my other amp, but Vce(max) is 120V, to low for this amp.

I've gone through the list and cannot source those transistors.

What transistors are current commercial amplifiers using ?

 

[CBS240]

I think you need to go with a small signal device and a few mA bias for the VAS and add another follower stage, a small signal pre-driver making the output stage a triple. After all, your driving 3 pair of output transistors...

 

[Ian Finch]

You can obtain various suitable VAS transistors around the globe that were once used everywhere as video drivers in CRT Televisions, as were the KSC3503/A1381 Fairchild clones.
RS and E14 have both reduced their through hole lines in recent years so I don't use them any more for anything other than large electrolytics, on which they seem to be out of stock with every purchase anyway. Manufacturers have a habit of dropping obsolete parts so, if you are spending considerable money on freight, as many outside Europe and the US do, buy a stock that will last for some forseeable years. They are cheap from Fairchild, so why not get as much as you can afford, as well as any other through-hole semis that will only become more difficult to source in the future.

It sounds expensive, but a $200 free freight order on Mouser or Digi-key gets many times the value of product that the equivalent in 5 or 6 smaller lots from RS or E14 does. This is how I buy most of my semis and specialised components now and I give those other guys a wide berth.

 

[AndrewT]

If you add a cascode to the VAS, then the cascode takes the big voltages and you can select a low voltage device for the VAS duty.
Low voltage devices have different compromises from high voltage devices and you should find it easier to get a suitable low Vce0 VAS device.

 

[djk]

Common base (cascode) may be the way to use the MJE340/350 and get good performance. It looks like they can do 60Mhz at currents around 10mA~20mA in common base mode.

The Musical Concepts replacement for the Hafler driver boards use the MJE340/350 in cascode around 2N5551/5401.

I wonder how a MJE340/350 cascode around a 125Mhz Darlington like the MPSA13/63 would perform? Mark Levinson did this with a 100Mhz Darlington and a 30Mhz cascode device.

The 340/350 and 13/63 are common industrial devices available in through-hole as well as surface-mount and should be widely available now, and in the future (because of their widespread use in industry).

 

[AndrewT]

I think you picked the wrong pair.
The 139/140 is the lower Cob and higher fT. Due to low Vce0 cannot be used in amplifiers with >+-40Vdc, unless some protection against over voltage is implemented.

The 300Vce0 mje340/350 is compromised to achieve that very high voltage capability. It is not suitable for VAS duty in conventional LIN topology.

 

[jaycee]

Ian, problem is CRT's are obsolete so the parts are being discontinued. Im quite surprised its taken this long - most CRT driver boards use an IC such as LM2402.

Commercial separates are often using LM3886 at the low end, or MJE340/350 based circuits. Mid range is now commonly becoming driver IC's like LM4702 or LME49810.

Suitable SMD parts exist but the problem is then power dissipation. So I guess cascodes are going to become the answer.

 

[AndrewT]

some smd have higher Pdmax.
eg.
To92 500mW
sot223 2000mW
for the same component.