Panasonic XBS mod

Hello everyone, i’m Rodrigo, and i live in Argentina. This is my first post, so hello to everyone. You have a very nice forum here!

I wanted to share with you my experience with the panasonic s-xbs in the year 2020, and ask you a few questions if it is posible.

I bought 2 panasonic walkmans recently. I changed the belts, and started playing old cassettes. I didn’t remember how annoying was this s-xbs Bass boost. The amount of bass is ridiculous. I remembered why I always prefered not to use it in the past. But unlike 20 years ago, now the service manuals are online for most of the models. I only know basic things about electronics, but i think i found a cap (surface mount, of course) that is responsible for that excess of bass. I soldered an ordinary cap in parallel (after testing a few different values) to the surface mount one, and for my ears, there is a big improvement in the sound. Now, when i turn the s-xbs on, there is more bass, but not too much, not annoying, not saturated, more defined low end. It really sounds much better. Now i always leave the s-xbs function active.

As i said, my knowdelge in the fields of electronics is very limited, and i don’t understand why there is less bass.

Here are the technical aspects:

I modified 2 walkmans. RQ-e30v and RQ-NX60v. Both share the same IC to control the xbs system. I searched in the manuals and found a pin of this IC1 that was labeled LPF (low pass filter i asumed!). Related to this pin, there was a cap (0.1uf or 0.22uf depending on the walkman model). As i said before, i tested different values in parallel with this cap, and noted the higher the value of the cap, the less bass i was getting in the earphones. That has no sense for me, because in a LPF the freq. is higher if the cap value is lower. Is it ok? Is my thought right? And if it is, why is there less bass in this circuit when the cap value is higher? Will my mod eventually blow the ic? For know, it has worked flawlessly. Can someone help me with this? I didn’t find any info in the web, so if someone can help i’d be very glad to learn what happened.
I forgot to tell you i’m using AKG-k72 headphones.
The images attached are from the nx60v.

Thank you very much!!!

Cheers

Rodrigo

 

Thank you very much!!

 

Hello Rodrigo and welcome to the forum.

A logical explanation would be that the original cap has lost its correct capacitance value.

If my calculations are correct :

- 0.1uf , cutoff frequency value is 35.37 Hz

- 0.22uf , cutoff frequency value is 16.08 Hz

If the capacitance value has become from 0.1uf to say 0.3uf then the cutoff frequency value becomes 11.79 Hz or 0.4mf to 8.84 Hz .

Not familiar with that walkman this concerns the LPF circuits in general.

 

Thank for your response Machaneus!!
I understand your theory, but i think that it cannot explain why adding a cap in parallel would attenuate the low fequencies, because when you use caps in parallel, the values in uf add to each other, for example:
Cap 1: 0.1uf
Cap 2: 0,33uf
Total c1+c2= 0.43uf
And as you said, the higher the value in uf, the more bass you may obtain, and that is exactly the opposite that happened in my circuit (i raised the value in uf and then the bass lowered).
My theory is that may be, the cap that i’m adding, is not exactly related with the LPF frequency. Maybe it is related with the amount of bass in the final signal?
I really don’t know. My knowledge is very limited...
Thank you again for your contribution.

As i’ve been suggested, i’ve already posted in the tech talk section my questions!!!

Cheers!
Rodrigo

 

For the sake of continuation I'll continue here.


Let's forget for a moment the term "bass" and talk frequencies :

- 0.3uf ( bad cap ) nothing passes above 11.79 Hz

- 0.1uf , nothing passes above 35.37 Hz this is higher frequency than the 11.79 Hz . The 11.79 Hz are still there but we hear also all the higher frequencies up to 35.37 Hz .
LPF 's sometimes referred as high-cut filters or treble-cut filters.

EDIT : Just to be clear this is not an answer to the problem you describing , 0.43uf should give
a cutoff frequency value of 7.37 Hz which is weird.

 

I agree it is very weird!
About the bad cap theory, if the stock cap (0.1uf acording to the service manual) was faulty in both of my units (strange but possible) , the new cap (0.33uf, you can see it in the pictures) may cut much more treble or high frequencies than the stock one.
I think our mistake here is that we are assuming the stock cap is related to the LPF. It may be part of another circuit or it may have another function not directly related with frequencies....
Even though, wondering about this problems is really fun. At least for now because my units are working nice!! I only wish i could understand a little more about electronics !
Thanks again for you help!!

 

... I found this to see what pin 8 does

 

My crude calculations are for reference only and took account only the 45kohm resistor in the IC and the fact that the sound is affected when a cap is added to the circuit.
Certainly it is more complicated , noticed that the LPF in discussion on IC pin 8 is actually LPF2 while on pin 11 there is LPF1 and they are related.

Not an expert myself , every day I learn something and this is indeed fun !

 

Thank you!!!

excellent!!! I think it will help a lot!! Now i’m at work, but later i will inspect it...

yes!!! You re right!! I’ve noticed there are 2 LPF pins (before performing the mod), but I First started playing with pin 8, and when it worked, i decided not to mess with the other one.... maybe LPF1 is related with the circuit while XBS is off? But i can’t be sure. I just decided not to take the chance to get the IC burned... I observed the performed mod in pin8 didn’t affect any frequencies when the bass boost is OFF. The mod ONLY affects the sound while XBS is ON.

Thank you all of you guys, i’m willing to keep on learning!!

 

Correct me if I'm wrong….So let's see if it is correct ... pins 10 and 12 audio input ... which in part goes towards the output amplifier and a part enters the low-cut filter ... the output of this filter is amplified and output from the pin 4 Bst Out and return from pin 3 to the output amplifier which is mixed with the input audio.

 

Yep it looks correct , (now someone to correct us both !) , thanks for the ride !

The amplifier/resistors setup of LPF1 , following pin 11, seems to be an active LPF (meaning amplification is used to maintain passive LPFs output , gain , etc disadvantages ).

 

I didn't comment before as I couldn't see where the other end of the capacitor was going, but in this schematic I can see that the other end of the capacitor (on this schematic C8) attached to pin 8 feeds through to the negative input of a a couple of amplifiers. Negative feedback will attenuate the signal. The other key pin is BST_NF which I would guess is short for Boost Negative Feedback.

For more understanding of how amplifiers are controlled using negative feedback have a look at

https://makingcircuits.com/blog/simple-bass-treble-tone-control-circuit/

This is an interesting IC doing similar things

https://learnabout-electronics.org/Downloads/lm1036-Tone control IC.pdf

 

Hello Longman, than you for your contribution! As I’ve told you earlier, It is hard for me to understand circuits , but i try, i read and i learn every day. So i need to clarify this:
Is it right to say that raising up C8 value would increase negative feedback over the amplifiers related to the LPF circuit, and that would explain why i hear less bass? If i am right: Which cap is responsible for the cut frequency of the LPF? C8? C12? Both?
thanks again, and sorry if my concepts are wrong. I have a lot to read yet about the subject!

 

Without a better datasheet or application note for the IC it is difficult to say, although I think you are right that increasing C8 increases negative feedback which means less bass. The actual cutoff frequency will probably be dependant on a different combination of capacitors and resistors. If you look at the circuits around 11, 12, 13, and 14 they look similar to the networks you get in the tone control examples in the first link.

Sorry i can't explain better but it is nearly forty years since I was learning this sort of thing at college.
After that I chose Digital Electronics as a career.

A little tip I do remember is that in normal operation the + and = inputs of an op amp have to have the same voltage. Otherwise the output will saturate. That is useful for DC condition calculations (which I actually used when fixing a complex record deck) but less so with AC.

Some basic theory on RC networks and how they are used to make filters.

https://www.electronics-tutorials.ws/filter/filter_2.html

 

Thank you Longman! I understand. It seems that even for people with solid electronics knowledge, it is difficult to get every detail clear without a proper datasheet.
Even though, i’m really pleased to see you all guys trying to help me! I’ve learnt a lot so far, and i hope i’ll keep on doing it.
If someone else has any other thoughts, i’ll be glad to hear!
If not, let me just say that i find it really weird, that no one else in the web has ever complained about the XBS bass boost. And that no one has ever tried to mod it. The only post about this topic (complaint) i found, is in this forum, and the author had some problems with certain types of music only.
Here is what he said:

Anyway, if someone ever wants to make some tests and has different or even the same results with this one or other mods to the XBS bass boost, i’ll be glad to hear about it, please let me know!
Thanks again!!
Rodrigo

 

I think most of us would either just turn off the XBS, or give up on the player completely. I did that on a disappointing Sony Headphone radio which seems to have Extrabass permanently enabled. Of course what you are doing is a great way to learn about electronics. A recent new purchase was a Pop Nano pocket DAB radio that has the opposite problem; no bass at all. A big disappointment compared to a more conventional radio of theirs I have. I am wondering if swapping some capacitors (e.g.output coupling) would solve that.

 

Very probalile both c8 and c12 .... but at this point I would also check the proper functioning of c3 which is located at the output on pin 4 ...

 

Me too Longman, i was one of those that used the walkman with XBS off. But what i mean is, nobody ever told to Panasonic that XBS was awful? After all these years? Really? Maybe with the stock (and nasty) earphones it was ok... I’ve read reviews in the web where people were saying XBS was wonderfull, even they said “XBS is a selling point”....

Thank you for the info!! I will keep away from that radio!!

Maybe in the future i will try more tests with c3.. why not? Thank you!!
Off topic: I’ve seen you are from Italy. You know that like a lot of people in my country, 75% of my family (grandparents) was from Italy, and 25% from Spain... ha! Yes, we are a blend!! Because of that, here there was a lot of Italian tecnology back in the early days of electronics... for example, i remember a portable open reel tape recorder: GELOSO was its brandname, i think it worked with a tube(12ax7 or similar)... you can still find them at sale in pages like mercadolibre (a Kind of local ebay)...
well, i don’t want everyone to get bored!!
Just thanks!!