F5 Listening Impressions & Discussion

[HVfanatic]

Hi Zenmod,

but: f5 for tweeters NEED low DC offset...

I'm still thinking about using 2 F3's, firing with a balanced pre, and employ a servo to keep both outputs on the same dc level... ;-)

Dirk

[Zen Mod]

Originally Posted by HVfanatic

Hi Zenmod,

but: f5 for tweeters NEED low DC offset...

I'm still thinking about using 2 F3's, firing with a balanced pre, and employ a servo to keep both outputs on the same dc level... ;-)

Dirk

we discuss that previously , I think ......

DC coupling of tweets isn't good , at least in my book - from two reasons which you already know - you need servo and you need anticlick ;

anticlick with speaker protection is must , at least for anyone having expensive/rare speakers

blablah ..... anyway - you already know that ....

I'm just puzzled about question - is series cap to tweet less evil than servo ?

be nice , and report your experience , when you try it

[hesener]

Originally Posted by HVfanatic

Hi Hesener,

I'm really looking forward to see your schematics... To me that looks much better than thermal compensation with NTC and unknown DC drift...

What I'm extemely unexperieced:

Compensation of amps. Do you have a good lecture (tutorial) on this???

Regs, Dirk

Hello, on compensation of opamps ("control theory") there should be a lot of material on the internet. sorry I dont have a tutorial at hand, but try Analog, Embedded Processing, Semiconductor Company, Texas Instruments, they have a great section on application notes, including a rather large text (600+ pages) on opamps, and I believe they talk about compensation as well.

And yes, simple circuits that work well have beauty in them, I think; I admit I also do have the NTCs and all the rest, but I was interested in doing it for the fun of it and so I did....
;-)

[bear]

Please keep in mind that the "normal" DC servo connection for "audio amplifiers" is back to the inverting node of the front end's differential pair. There is no differential pair in the F5.

_-_-bear

[AndrewT]

Originally Posted by bear

DC servo connection for "audio amplifiers" is back to the inverting node of the front end's differential pair. There is no differential pair in the F5.

two solutions and probably many others.

1.) take the non-inverting DC servo output to the source of the input FETs, just like NP does with the 50r feedback resistors.

2.) use an inverting DC servo and take the output to the input FET gate.

[bear]

Returning a DC voltage to the input JFet gate would not be a good idea since the input is not AC coupled.

Andrew, did you say if you have built an F5 yet?

_-_-

[hesener]

Sorry for the delay, as announced last week here is the schematic for the F5 with servos.

Couple comments:
>> Two LM2904s are needed, since their max VCC is 26V, and I had to get the lower close to the source potential of the n/channel power MOS. It is important to use an opamp that will allow for the inputs to be able to go down to its negative rail (this opamp has PNP inputs so no problem).
>> In this schematic, the output overcurrent protection is still in there, but the NTC is not (no longer needed....)
>> The lower circuit uses a TL431 to create a temp-compensated reference voltage, against which the voltage on the source resistor of the n-channel power MOS is compared and regulated accordingly - no real magic here. The upper circuit compares the output voltage to ground, and regulates the p-channel power MOS accordingly - no real magic here either.
>> R41 and R42 can be replaced with a trimpot to adjust the bias current.
>> Note C96 - this is to "disable" Q11 working as a grounded-base amplifier (the original circuit is pretty immune to (50Hz) noise on the rails, the better the gain between the two halves is matched the lower the noise. this circuit injects a lot of noise without C96 - with a regulated PSU you probably won't need it)
>> The caps on the rails are just for local bypassing - tried the amplifier with a switchmode lab supply (and long cables) and it oscillated nicely at 300kHz..... the caps made that disappear immediately . I put +/-20V but it doesnt really matter.

hope you like it!

still need some time to build a couple channels with the original circuit and compare the sound.....

[sekess]

Once again -- very impressive Hessner.
Where did you mount the components -- on a seperate board?

Steve

[bear]

Q: U1A and U2A are unused?

_-_-

Btw, just thinking aloud, it would seem to be sufficient to keep one side manually adjusted to bias up and then servo adjusting the other to make 0vDC offset?

[a.wayne]

And adding the servo circuit improves the sound ....... How ? .......