DIY JE-990 DOA

[bcarso]

[quote author="skipwave"]However, with C3 between ground and RV1 (gain set pot), isn't this current feedback mode? Yet, there is a bandwidth limiting cap, C2, across the feedback resistor, R3.
[/quote]

External passive components do not a current mode amplifier make. The 990 is a conventional voltage-feedback opamp.

Current feedback amplifiers have a low-impedance inverting input whose voltage typically tracks with about unity gain the voltage at the noninverting one. The current flowing in the inverting input is conveyed to a high-Z output node via current mirrors or current amps, and buffered by a unity-gain output stage.

 

[skipwave]

Thanks Brad!

That being the case, what would motivate a designer to move the blocking cap between the gain set resistor and ground, as opposed to between the inverting input and the resistor?

 

[Samuel Groner]

Where's the start of a pot and where's the end..? And what makes you feel like the cap should be at the other end? Without claiming statistical significance I believe that I've seen far more schematics grounding the cap (i.e. as shown) instead of moving it closer to the inverting opamp input. May give a slight improvement with respect to noise pickup as the large capacitor sees at least at one end a low impedance; the main reason is probably just visual clarity though--it seems much more logical to me to have the cap at ground instead of seeing it float somewhere else.

Samuel

 

[skipwave]

Oh, I remember now I was thinking about the discussions I've read before regarding the possible evils of having any DC offset voltage applied to the pot. A switch with resistors avoids that issue, though it was debated whether or not it was even a significant one.

Apologies for the diversion, I'll let this thread get back on track now.

 

[rodabod]

If anyone in the UK struggles to find decent inductor wire, I can send them some.

 

[clintrubber]

[quote author="Samuel Groner"]it seems much more logical to me to have the cap at ground instead of seeing it float somewhere else.[/quote]
FWIW, likewise for an eventual output-cap: it can be 'below' the output-TX primary as well
(so amp-output directly to the prim-top, and cap between prim-botton & gnd).

Regards,

Peter

 

[clintrubber]

[quote author="skipwave"]A switch with resistors avoids that issue, though it was debated whether or not it was even a significant one.[/quote]
A non-shorting gain-switch might cause pops, in which a large value resistor across somewhere can help. Or a shorting switch to begin with.

Apologies for the diversion, I'll let this thread get back on track now.

Don't apologize, all relevant for the use of this opamp I'd say :thumb:

Regards,

Peter

 

[rodabod]

Does anyone know if Q10 (the diode connected transistor) needs to be matched in any way with Q3 which it is thermally-coupled to?

John Hardy has labelled this as a selected part in his 990 pdf which did make me wonder. At first I was going to match them for HFE before realising how it was configured.

Roddy

 

[Samuel Groner]

Ideally they would be matched for Vbe at the right collector current. Not super-critical though. Matching for hFE doesn't help.

Samuel

 

[Soeren_DK]

I have some problems with the 990 DOA.

I have build two API clons http://www.groupdiy.com/index.php?topic=32028.0 with the prebuilds mnats 990 DOA.
One of the boards is working very well but the other have some problems.
The powersupply is okay. I have checked the voltage and its -+24V.

The R7 resistor on the DOA started to burn and I turned it off. I can't see any problems on the PCB but maybe you out there have an idéa what to look after.

??? ??? ??? ??? ???

Cheers
Soeren

 

[3nity]

make sure all the transistors are in their right place! and all resistors are good!
Thanks.

 

[Soeren_DK]

I have checked all parts on the board and they seems to be okay. I replaced the R7 and then it suddenly worked. I tested it for a little while but then I could hear some loud crackles and then it started to smook again...

Any ideas what to look after?

Cheers
Soeren

 

[Black Dog]

Any ideas what to look after?

Cheers
Soeren

The mailman - I'm sending you another couple of 990's..

Lars

 

[Ptownkid]

Nice!

 

[brazilianwonderboy]

I have checked all parts on the board and they seems to be okay. I replaced the R7 and then it suddenly worked. I tested it for a little while but then I could hear some loud crackles and then it started to smook again...

Any ideas what to look after?

Cheers
Soeren

sbranco had a similar problem on around page 6 or 7 of this thread. Take a read of it and see if that helps.

 

[SIXTYNINER]

HI
THERE IS SOME JE 990 KITS AVAILABLE
SOMWHERE?
thanks

 

[seavote]

ptownkid usualy has them on his web site. he's not offering any now but probably will in the future.

http://www.diypartssupply.com/

 

[janczmok]

Hi,

does anybody got 6 pcs. of the 62pf caps ? i am not able to source those for my 990's.

cheers

jan

 

[juniorhifikit]

I understood you both have a six-hole core Mnats990 and a Toko-style Mnats990, right ? Was just curiuous if you'd had put an ear to those for a first impression.

Naw, I'm just fooling around with the inductors until I decide what is going to go in there. The six-hole core I have probably won't go in because it just looks wrong with some windings doubled up in two of the holes. All I've measured with the bobbin-style is the inductance; I have no idea how I'm going to measure the other parameters with the limited equipment I have.

...

Seems like the Toke does the trick

Hey, good idea!

I haven't read through the whole thread, so forgive me if this has already been covered, but Jensen sells the 6-hole inductor cores by the dozen (which they buy from Fer-rite).  Why not just buy them from them?

 

[dmp]

Thanks to everyone who made this project and figured out the tricks in these threads. I built up 4 990 DOAs and they all worked great right out of the gate.
I used smd caps for the 62pf (Mouser: 81-GRM215C2A620JZ01D). These weren't terribly difficult to solder in, just do it before you've had too much coffee
Search for a multilayer ceramic, NGO / COG, 62pf
For instance, Farnell # 1758963