WM-EX60 dynamic bass boost tuning

Hi everyone,

My name is John. I recently purchased a WM-EX60 in mint condition. It is fully functional, and I tested it with several recent albums on cassettes from 2022-2023. The cassettes have good recording quality, tested on other decks. There were no original headphones for the player (old and bad anyway), so I connected my over-the-ear Sennheiser HD 490 Pro headphones and in-ear AKG headphones. I know for sure they both can provide amazing sound from a quality audio source.

Back to WM-EX60. I immediately noticed the poor bass/treble tuning by Sony engineers back in the 90's. I tried all three positions of the EX-DBB switch: NORM - no bass and no treble. MID - overboost bass, almost normal treble, MAX - crazy bass overload, unbearable distortion throughout the entire frequency range of music.

Electronics is my specialty. So, I took the circuit diagram of this player and traced the path of sound from the tape head to the headphones. Notes: Dolby B NR disabled.

I found the NJM2106M (EX-DBB) halfway from preamp to power amp. I downloaded the datasheet from the New Japan Radio Co. and studied it carefully.

Part of the original circuit for the EQ:


Here are my notes:
Sony engineers deviated from NJM's recommendations and adjusted the bass boost in a strange way. They extended the frequency range of the bass boost from the proposed sub-/low-bass range (18Hz/3dB cutoff) to the mid-bass range (122Hz/3dB cutoff). Someone said that headphones of that time lacked sub-/low-bass response, so they decided to increase this range up to +20 dB (dynamically variable). In my opinion, this is too much. This is not a problem for modern headphones, which do not lack bass range. However, boosting the mid-bass range results in a muddy sound on new recordings where the required frequencies have already been mastered before recording. I decided to tune the EX-DBB circuit closely to that described in the NJM datasheet, with my own adjustments. My goal is +6 dB sub-/low-bass boost and +6 dB treble boost.

Recommended settings from NJM datasheet:

As I see it, I need to replace the C307 0.01uF capacitor with a 0.068uF one to lower the low pass filter (LPF) cutoff frequency from 122Hz to 18Hz/3dB. Then replace R305 2.2KOhm with 4.7KOhm to reduce the HPF amp gain with weighting function from +10dB to +6dB. And lastly, change the values of capacitors C113/C213 from 680pF to 820pF to reduce the pass frequency of the high-frequency amplifier from 12 kHz to 10 kHz. The high frequency gain is almost fixed by the parameters of the NJM2106 internal resistors, and is in the range of +5...+6dB for 20 kHz, which is enough for me.

I have pro soldering equipment for working with SMD printed circuit boards. As well as a multifunctional frequency generator, spectrum analyzer, oscilloscope, power supply and other lab equipment. I think it won’t be difficult to test the resulting changes in bandwidth and gain for this circuit.

Any comments, suggestions, please share.

 

So, here are our replacement candidates:



My caliper shows that the SMD component sizes are close to the 0603 imperial (1608 metric) package size. This is what I ordered from a local electronic components supplier.

 

I don't have the model device in question. But this entire subculture community of old Walkman repair, restoration, enhancement etc,. needs more individuals with your abilities and skills. So I consider any good analysis, like that which you just provided here, as being a very important and useful contribution. In my opinion.

If this modification does indeed work as anticipated then one would expect such to also apply to other models with same / similar design compromises. A lot of questions and potential is then opened up for other models. I would suspect anyway. Very cool. So please stick around and don't get discouraged!

 

Thank you. I quickly looked at the circuit diagrams of other Walkman models and found that most EX-DBB models have a similar circuit with a 3-way switch (NORM, MID, MAX). And most MEGA BASS models have an EQ function built into the preamp chip with switch control (MEGA BASS ON-OFF) or the latest models with a switch and BASS/TREBLE potentiometers (better, IMO). The way EX-DBB is implemented is useless with current cassette recording and headphones.

While I'm waiting for parts from the supplier, I ran a math simulation of the frequency response of this circuit.
The NJM2106 datasheet suggests the following settings:

Legend: Green - Low Pass Filter; Orange - High Pass Filter, Blue - Weighting Amp, Purple - Total.
Peak gain is +11 dB at 20Hz, and bandwidth is 6-60Hz/3dB.

Factory settings of the EX-DBB in WM-EX60:

Peak gain is +10.5dB at 50Hz and bandwidth is 10-140Hz/3dB.


Peak gain is +12dB at 50Hz and bandwidth is 10-150Hz/3dB.

I think they should have made the MID boost +6dB and the MAX boost +10~12dB.
Instead, they set the MID gain too high, making it unusable and the MAX unbearable.
And they moved the peak gain frequency higher, widening the bandwidth, which bleeds into the low bass region, making the music sound muddy.

 

And this is the hypothetical response of the circuit to my tunings with changes in the resistors and capacitors mentioned above.

Now all that remains is to wait for the arrival of all electronic components, replace them on the printed circuit board and perform listening / instrumental measurements.

 

Yes indeed. MID is too high and MAX is unusable.

 

John, I think this is great, that you're looking into this sound modification on this model(EX60). The WM-EX60/EX70 are my favorite! I like the looks of the Clear"Round" window on EX60! They Play nice and Sound nice, but yes "Normal is where I keep the settings, but sometimes it sounds to "Flat".

Can't wait for your results on this!!

 

Thanks, Duey71!

I'm waiting for electronic parts that I calculated based on simulating the response of the equalizer control chip. They are supposed to arrive next week so I will have a chance to replace them.

But before that, I would like to conduct frequency response measurements using a signal generator and a magnitude / spectrum analyzer. Kind of, before / after. This way, I should have quantitative graphs to prove the changes I made and their impact on the output sound. The reason for this is the NJM2106M's non-linear response to input signals. This acts as a "loudness" function of some audio devices, amplifying low amplitude signals more than high amplitude signals over a certain frequency range.

Also, I looked at the circuit diagrams of the latest Walkman models from 1999 and 2000 (WM-EX900, WM-EX20), and noticed that they had a physical bypass for disabled features (Dolby NR, EQ Surrond) using transistor switches in both stereo channels. Thus, they eliminate any possible audio degradation along the signal path that may be caused by these chips.

I was thinking about this for the EX60 as well because we have two of these chips in between the tape head preamp and the headphones power amp. Even if we turn them both off (Dolby NR off, EX-DBB normal), the audio signal has to pass through both of these chips and there is a chance that they will cause some degradation in its quality. I'll measure / listen to this by manually jumping the audio signals around one or both of them to see their effect in off states.

And also the value of the decoupling capacitors in these circuits. In EX60 they installed 4.7 µF, but in subsequent models 6.8 µF. This may also be part of the problem. I think it should be at least about 10 µF.

 

The other day, I received all the parts and test tape from four suppliers. I hope to start these things in the coming days. First, I will measure the current wow and flutering numbers, then replace the belt and pinch rollers and repeat the measurements. Secondly, I will make initial measurements of the frequency response of the NJM2106 EX-DBB using the original circuit.

 

Performed measurements of W&F and tape speed followed by replacement of the belt and pinch rollers.
Before: W&F 0.9-1% DIN, tape speed 2860 Hz on test tape 3000 Hz.
After: W&F 0.45~0.5% DIN, tape speed (adjusted) 3000 +/- 15 Hz.

W&F of 0.5% seems too high to me given the new belt and pinch rollers. I can't find the factory W&F specifications for this Walkman model.
The service manual contains only the following information:

Does anyone know if +/- 15Hz of the reference frequency 3000 Hz means (15/3000) * 100% = 0.5% factory specs for W&F?

I have not yet done any disassembly to lubricate the mechanism. To do this the PCB needs to be removed and before removal I want to perform a frequency response test on this board.

 

Typically, most belt walkmans don't have specifications for w&f.
The measured figure is indeed way too high, something around 0.2%-0.3% WRMS being desirable.
By DIN I assume you're reffering to the WFGUI DIN readings which are WRMS.

Would be curious what the value is after a re-lubrication of the mechanism.

As for the +/-15Hz, yes that represents 0.5%.

 

That's right, these are the RMS% DIN values from WFGUI 8.60. I'm also disappointed by such lousy results with the new belt and pinch rollers from fixyouraudio. I know that these are aftermarket parts and the quality may be lower than the original ones. I'm also not sure if lubrication can make a big difference, because everything can be easily turned with a finger.

 

Okay, today I found time to unsolder the coupling capacitors C112, C212 from the input side of the NJM2106M chip for both stereo channels. Temporarily soldered two negative sides of 10 μF capacitors to pins #1 and #16 of the microchip. Extended the positive terminals of the capacitors in a twist with a common wire outward, for connection to a bench multifunctional signal generator.
Hopefully I'll have time to do some sweeps and pink noise signal testing in the coming days to determine the current frequency response of the circuit with all three EX-DBB modes.
I'll post here some graphs from the spectrum analyzer.

 

OK, I'm getting confused again with the

W&F results WFGUI V8.6 I just read my perfect lubricated EX60. 3000Hz My RMS results are 0.6671 and 0.5152 (Peak at 1.030 or below) this is what I normally been seeing with my 3 WM-EX60 and EX70. They sound great what I listen too. That's why I said it was normal on these? Valentin is saying it's to high? I'm lost, I'm not sure how to get any lower. The Best is 0.4231 RMS between all of my EX60, EX70's.


I thought I better post, because I told Johny .5 was good. Now I'm Lost. I'm not the Expert either. I don't want no enemies! Just some new Cassette Friends..

"Ok, My Best one gets %RMS 0.4261(PEAK = 0.848)"

-Duane

 

Thank you, Duane.

I am getting ~0.5% RMS with new drive belt and pinch rollers from fixyouaudio.
So far I've only cleaned everything on the tape drive with isopropyl alcohol.

After I have made measurements and adjustments to the electronics, I will unsolder the main board and start cleaning and lubricating the rest of the mechanism.

 

My source of parts are from the same source. So, these are the typical values I get from this series. I was just trying to make sure there were no confusion or conflicts. Why I took a photo to just show my average and setup. I'm waiting on your Sound Lab Results!!

 

Wow&flutter of 0.5% WRMS is high. While it can still be acceptable with some kinds of music, it will be audible during passages with a bit longer note decays.
Unfortunately I'm not familiar with these EX60/70 mechanisms, so can't say what can be done (if anything) to lower the w&f figure.

The first thing I would check is the FWD figure vs REV figure. If the FWD figure is significantly higher it means w&f is caused by something in the geartrain, possibly a worn driving gear on the flywheel.

The belt itself will also play a role, in the sense that there is a certain manufacturing tolerance.
It is also possible this mechanism is very sensitive to the belts used. Worth trying another belt from DeckTech in this regard just to see if there is a difference.

 

I decided to do the electronic tuning first and then do the mechanical tuning. The PCB is in the access path to the gear drive for maintenance, so I try to minimize the amount of desoldering of all flex cables in this process. Luckily, the belt and rollers are replaceable without removing the circuit board, so this was done right away.

The current 0.5% RMS sounds good to untrained listeners with regular pop/rock cassettes.
Tested on my family members . The only time I notice W&F is the long monotonous sound in recordings. For example, a 3000 Hz speed test cassette has a noticeable effect. I have a recording before belt/rollers replacement with 1% RMS and after replacement with 0.5% RMS. Tomorrow I'll post these files here from another computer.

 

Thank you, Valentin. I thought so too. I'll take a chance and buy a couple more belts from DeckTech to see if there are any differences.

 

Ok, keep me posted on the DeckTech Belts and results. On Piano music I hear the difference in W&F and on various David Bowie. This is where the DD series start to shine on stable playback. .

Thanks Valentin for all your input on this. I try myself not to tie all my time in W&F #'s unless troubleshooting, etc. It's a hobby in itself.