It's been a long time since i last posted here. I'm building this simple amp circuit with Jfets and a TDA chip amp. I want to put a negative feedback loop there to stabilize the sound and tame it a little, make it sound more like a Fender. How would i do that? And more importantly, do i even need that in this type of circuit? Below is a simple layout. Thanks
https://i.postimg.cc/prqk9zbk/IMG-20241022-214315.jpg
(https://i.postimg.cc/prqk9zbk/IMG-20241022-214315.jpg)
JFETs are configured as CD or CS amps..??
Quote from: antonis on October 22, 2024, 04:12:42 PMJFETs are configured as CD or CS amps..??
Common source
2nd JFET Source to 1st JFET Drain or 2nd JFET Drain to 1st JFET Source..
Why are you powering the JFETs with half the voltage? JFETs have no (little) gain. You want the most you can get, even if you want clean tones.
Quote from: seadi123 on October 22, 2024, 03:57:46 PMI want to put a negative feedback loop there to stabilize the sound and tame it a little, make it sound more like a Fender. How would i do that? And more importantly, do i even need that in this type of circuit? Below is a simple layout. Thanks
The TDA7052A already has it's own internal feedback.
Perhaps what you want to do is try a master volume idea between the last JFET and the TDA7052A. Once you determine what you want you could replace that with a simple fixed resistive divider.
Quote from: antonis on October 22, 2024, 04:34:33 PM2nd JFET Source to 1st JFET Drain or 2nd JFET Drain to 1st JFET Source..
That would put the tone stack inside the NFB loop?
Quote from: merlinb on October 23, 2024, 05:01:23 AMQuote from: antonis on October 22, 2024, 04:34:33 PM2nd JFET Source to 1st JFET Drain or 2nd JFET Drain to 1st JFET Source..
That would put the tone stack inside the NFB loop?
Definitely.. :icon_wink:
That said, we'll have a much less effective Tonestack due to NFB loop compensation..
P.S.
I should to not reply in late hours.. :icon_redface:
Quote from: fryingpan on October 22, 2024, 05:14:05 PMWhy are you powering the JFETs with half the voltage?
This.
Quote from: PRR on October 23, 2024, 04:54:24 PMThis.
Even more than that is we know nothing about the JFET stages.
How much gain do they have? Do the JFETs have source resistors? are they bypassed?
If the first stage clips it's all over if you want a clean tone as an option.
I didnt include the schematic for simplicity sake. Its basically 2 fetzer valves with a bassman tonestack in between them, followed by the TDA chip. Schematics below:
Fetzer Valve (https://i.postimg.cc/DwkgbV7D/fetzervalve.png)
(https://i.postimg.cc/DwkgbV7D/fetzervalve.png)
Bassman Tonestack (https://i.postimg.cc/9f1tSry5/tonestack.png)
(https://i.postimg.cc/9f1tSry5/tonestack.png)
The Fetzer Valve has very low gain. With a J201, at 9V, you're looking at about 6dB per stage. (At 4.5V, you're looking at negative gain :D ). Whatever tonestack you will use is going to introduce at least 6-10dB attenuation. The Fender tonestack is more like 16dB attenuation (going from memory). Basically you're using up the two JFETs to partially compensate for the insertion loss. :icon_razz: Also, a J201's average Vgs(off) is in the vicinity of 0.9V. With hot pickups you are going to clip, hard. It might not matter (it's just on the transients, after all) but if you want clean gain, unless you use a higher Vgs(off) part (like, for instance, the 2N548x series) *and* higher voltages, the Fetzer Valve is not going to give you it.
Quote from: fryingpan on October 24, 2024, 05:21:43 PMThe Fetzer Valve has very low gain. With a J201, at 9V, you're looking at about 6dB per stage. (At 4.5V, you're looking at negative gain :D ). Whatever tonestack you will use is going to introduce at least 6-10dB attenuation. The Fender tonestack is more like 16dB attenuation (going from memory). Basically you're using up the two JFETs to partially compensate for the insertion loss. :icon_razz: Also, a J201's average Vgs(off) is in the vicinity of 0.9V. With hot pickups you are going to clip, hard. It might not matter (it's just on the transients, after all) but if you want clean gain, unless you use a higher Vgs(off) part (like, for instance, the 2N548x series) *and* higher voltages, the Fetzer Valve is not going to give you it.
i am actually planning on using 2n5457s cause j201s are hard to source. im not very concerned about volume, id rather make it as quiet as possible, im even considering removing the second fetzer valve stage, how bad would that be? Also since the chip apperently has internat negative feedback, i will be omitting the idea of inserting one since fender amps dont have a netagive feedback loop in the preamp anyway
Quote from: seadi123 on October 24, 2024, 06:15:50 PMQuote from: fryingpan on October 24, 2024, 05:21:43 PMThe Fetzer Valve has very low gain. With a J201, at 9V, you're looking at about 6dB per stage. (At 4.5V, you're looking at negative gain :D ). Whatever tonestack you will use is going to introduce at least 6-10dB attenuation. The Fender tonestack is more like 16dB attenuation (going from memory). Basically you're using up the two JFETs to partially compensate for the insertion loss. :icon_razz: Also, a J201's average Vgs(off) is in the vicinity of 0.9V. With hot pickups you are going to clip, hard. It might not matter (it's just on the transients, after all) but if you want clean gain, unless you use a higher Vgs(off) part (like, for instance, the 2N548x series) *and* higher voltages, the Fetzer Valve is not going to give you it.
i am actually planning on using 2n5457s cause j201s are hard to source. im not very concerned about volume, id rather make it as quiet as possible, im even considering removing the second fetzer valve stage, how bad would that be? Also since the chip apperently has internat negative feedback, i will be omitting the idea of inserting one since fender amps dont have a netagive feedback loop in the preamp anyway
2n5457s (that was the series I intended) would basically have 6dB of gain at 9V. (I double-checked and J201s would give you double that, which would be more workable but, again, less headroom). It is true that the TDA7052 has a fixed gain of 40dB, which means that you probably can get away with no gain *at all*, but with the Bassman tonestack (as I said, 6-12dB insertion loss on average, depending on settings - you could rework the values to reduce insertion loss by 2-3 dB) I still would make sure that those 40dBs the chip amp provides are all there to be used, and I'd imagine some leeway as well (let's say 10dB).
At the bare minimum, I would include the option to bypass the source resistors (you'd get about double the gain from each stage).