strangeandbouncy
Well-known member
Anyhow . . . .
moving rapidly on . . . .
moving rapidly on . . . .
ours sound like the 2*5*2*0?
- Thread starter Mbira
- Start date
Help Support GroupDIY Audio Forum:
strangeandbouncy
Well-known member
But seriously,
would you mind if I popped round at some point to scan some stuff? I only (officially) work 3 days a week, and have to pop back to malvern rd to check on da builderz. Whats your schedule like? My wife works just up the road on Sat mornings and i generally give her a lift to work if that is cool with you(?)
Small world!
Andy p
would you mind if I popped round at some point to scan some stuff? I only (officially) work 3 days a week, and have to pop back to malvern rd to check on da builderz. Whats your schedule like? My wife works just up the road on Sat mornings and i generally give her a lift to work if that is cool with you(?)
Small world!
Andy p
G
Guest
Guest
[quote author="strangeandbouncy"]How Bizarre!
- So's my flooded flat . . . . Malvern Rd, me . . . . . behind Chippenham pub.
Andy p[/quote]
Can I join this love in....
Problem is I seem to have missed it by 20 years..
I used to live in a grotty bedsit in Fernhead Road....(between the Harrow Road and Malvern Road)
I am following this thread with mucho interest....
Boys get to it...
- So's my flooded flat . . . . Malvern Rd, me . . . . . behind Chippenham pub.
Andy p[/quote]
Can I join this love in....
Problem is I seem to have missed it by 20 years..
I used to live in a grotty bedsit in Fernhead Road....(between the Harrow Road and Malvern Road)
I am following this thread with mucho interest....
Boys get to it...
strangeandbouncy
Well-known member
Da Arry,
check your pm;s . . . .
I have been tied up at studio. Mixing Mod-Aid record. PP Arnold, Chris Farlowe, Ronnie Wood, Reg Presley and many more . . . . Great Laugh.
Should be in da Hood early next week, fixing sessions tomorrow. Call you tomorrow if that's ok with you?
Andy P
check your pm;s . . . .
I have been tied up at studio. Mixing Mod-Aid record. PP Arnold, Chris Farlowe, Ronnie Wood, Reg Presley and many more . . . . Great Laugh.
Should be in da Hood early next week, fixing sessions tomorrow. Call you tomorrow if that's ok with you?
Andy P
strangeandbouncy
Well-known member
Will Do!
Andy p
Andy p
I've been very busy lately. However I've managed to get my api/what/Pultec/QE-combo to pass some sound but it still has some hum issues. The preamp stage has actually a couple of relays to switch between two op-amps so I can have two different mic pres in one box.
I was pretty surprised to find out how similar the two op-amps sounded at first. First I could hardly find any difference between JE-990 and a 2,5,2,0 and thought that I must be deaf or something..
After listening a while I started to hear the difference. They both have a character of their own but it's surely not like a neve vs. api kind of difference.
It's great to be able to compare op-amps with a click of a switch and it tells something that could not be learned otherwise.
I' still don't have the soundclips that tells the whole story but they will be here soon.
cheers
-Okko
Neeno
Well-known member
[quote author="deveng"]I've been playing around with the tail current values on the 1731 and found some interesting things happening. First, at lower gains typically used on mic preamps with condensor mics (10-20db) the Pspice plots show a peaking at 1.5Mhz. According to Doug Self's discrete op-amp design webpage: http://www.dself.dsl.pipex.com/ampins/discrete/discrop.htm
He is referring to the ratio of value of the resistors on the collectors to the tail values. If I reduce the 1731 schematic tail resistor value to about 20k the bode plot smoothes out very nicely. I see a similiar thing happening on the 2*5*2*0 circuits when the current set by the current source is too low. Doing a bit of research it appears that small signal transconductance is proportional to tail current making the circuit non linearly voltage dependent. I'd like to see someone with PRR's design experience and intuition jump into this one.
If some of the schematics floating around don't have these values set properly, it may explain why they may not sound as good as they can (or closer to the real thing). Bench testing with the proper equipment may reveal some even more interesting things.
Comments anyone....
regards,
Jeff[/quote]
I've finished two melcors last night... and i got the "problem" you're talking about... a peaking @ +- 1MHz at lower gains using condenser mics... i tried different feedback caps, but no way to solve it... have you any suggestion/solution to solve this issue ?
Neeeeeeeeeeeeeno
He is referring to the ratio of value of the resistors on the collectors to the tail values. If I reduce the 1731 schematic tail resistor value to about 20k the bode plot smoothes out very nicely. I see a similiar thing happening on the 2*5*2*0 circuits when the current set by the current source is too low. Doing a bit of research it appears that small signal transconductance is proportional to tail current making the circuit non linearly voltage dependent. I'd like to see someone with PRR's design experience and intuition jump into this one.
If some of the schematics floating around don't have these values set properly, it may explain why they may not sound as good as they can (or closer to the real thing). Bench testing with the proper equipment may reveal some even more interesting things.
Comments anyone....
regards,
Jeff[/quote]
I've finished two melcors last night... and i got the "problem" you're talking about... a peaking @ +- 1MHz at lower gains using condenser mics... i tried different feedback caps, but no way to solve it... have you any suggestion/solution to solve this issue ?
Neeeeeeeeeeeeeno
deveng
Well-known member
It's been awhile since I played with the simulations for the 1731 but as I remember, lowering the tail resistor value would solve your problem. By dropping the value of the tail resistor the tail current increases. This does effect the upper end freq rolloff and did smooth this 1.5Mhz bump. I simulated down to a 20k resistor (about 120ua of tail current) which seemed to work well for this offending peak.
Which combinations of feedback resistor and capacitor did you use? This would also reduce this peak although not target it directly since the pole will be at an entirely diffenent frequency (much lower). If you're using the typical 20-25k feedback resistor then I suggest moving up to a 100-120pf polystryene. You can probably go as high as 200pf if your feedback resistor is on the lower side. Don't go any higher than you absolutely need on this capacitor.
The values of the resistors on the collectors is also a concern of mine. I haven't done much with changing these but I have a feeling this might be something to look at as well. Although, I don't believe it will help your problem so don't change these just yet.
I'll try and load the simulations up for the 1731 again and verify this advice and see if there are other options. The fact that you had this problem is both good and bad. First, it shows that Pspice simulations are valuable and can predict possible problems before assembly. Although, as PRR mentioned, the results from Pspice should always be carefully considered and not taken as fact. The bad part is that you've got to make changes and deal with the problem. But, this is all part of the fun!
Regards,
Jeff
deveng
Well-known member
Bauman:
Yes, but I was in error about the value.
Neeno:
Memory didn't serve me well. 20k is too low. DON"T USE THAT VALUE! But, you can go down to 120k (which should increase the tail current to about 120ua). But, it won't really sovle the 1.5Mhz bump.
I re-ran the simulations and found that the bump at 1.5MHz (low gains) was always there. In this short amount of time I haven't been able to find the solution. But, the use of a higher value of feedback capacitance ( in the order of 120-200pf or so will tame it. What are the values you use now? The frequency response of the input transformer should keep any of this frequency from hitting the op-amp directly. Good layout on the pcb at the op-amp inputs and short runs for the feedback loop should keep external sources away. If you still have the bump it's probably an oscillation problem that may not be solved by the methods mentioned here.
I'll keep plugging away at the simulations to see if I can find a simple solution.
Regards,
Jeff
Hi All .I´m building my first 2520 opamp and have some questions:
Should I connect two diodes in series on D1 ? Do I solder r15 and c3 together on the upper side ? Should I use two resistors on r4+r5 to sum the value (90k)or should I use one resistor ? I´m using Fabios 2520BCr.1
Thanks
Should I connect two diodes in series on D1 ? Do I solder r15 and c3 together on the upper side ? Should I use two resistors on r4+r5 to sum the value (90k)or should I use one resistor ? I´m using Fabios 2520BCr.1
Thanks
Hey,
Yes, 2 diodes as D1. No you should solder R14+C3 and then solder each side to the board. R4+R5 must be like 20k or 22k, and make R3 the same.
With 82k as shown it doesn´t work welll...
I did some metallic cover to this little things....
Now I need some epoxy :twisted:
cheers!
Fabio
Yes, 2 diodes as D1. No you should solder R14+C3 and then solder each side to the board. R4+R5 must be like 20k or 22k, and make R3 the same.
With 82k as shown it doesn´t work welll...
I did some metallic cover to this little things....
Now I need some epoxy :twisted:
cheers!
Fabio
