Let's be honest...we really get the most out of our boomers when hooked up to an AC source. Not a surprise there.
However, there are some manufacturers who will put a more expensive transformer in their product.
For example:
My Telefunken HiFi Studio-1, Hitachi J3, Telefunken CC9000 all use the highly regarded "R-Core" transformer. Of course the size of the transformer in the Studio-1 is ridiculous for obvious reasons.
And others like my Hitachi TRK-9900 use a torroidial transformer.
Typically what you will see in boomboxes are the following EI types...
I have (or have had) a unit that used a C-Core (I know I saw it but can' remember which it was.
)...
In a good chunk of my "Heavy Hitters" the two former I see more than the latter. Here are the major differences with these types starting with the low cost massively used EI...
1. EI Type - It's made of an E-shape lamination and an "I" shaped lamination and very easy to wind accurately. Commonly used for more than 100 years due to the easy workability, it can be wound in very complex designs. But there is a cut-end on them and magnetic leakage loss is high. Interference and noise are high too. There are still many power transformers, choke, output and IT using this format as it is still the most cost effective to date.
2. C Core - A very rare but great core. There are double C cores and normal C-cores. Normal C-cores just looks like an R-core cut into 2 pieces. The cut edge is an air gap to prevent core saturation. It also allows the winding to be done externally. So, it has both advantages of EI and R-core. It has a slightly lower efficiency than R-cores but much better than EI. It was commonly used for many years in higher end equipment as an audio PT, choke, and OPT and choke. Making a C-core is much harder than a EI and therefore typically cost prohibitive for mass production on very large scales.
3. Torroidial - The core is a roll of long thin ferrous sheet. Since it has almost no cut edge, the magnetic loss is near zero. Also, noise and interference is very low. It can be placed very close to amplification parts without the need for heavy shielding. It is extremely suitable for compact designs as it also has very high efficiency and low heat generation. Because the windings are affected due to the inner core and outer core being a different size, the windings are not as accurate or linear and can affect sound in audio applications where used downstream in a circuit. Winding of torroidials is probably the most difficult of all the cores mentioned. Because of the issues with linearity these are used generally as power supplies mains only. Because there are no gaps in the windings torroidials are more susceptible to having their ability to pass some AC power limited by by residual DC currents in the windings.
4. R Core - These were developed by the Japanese as an improvement on the torroidial. Windings are only applied on the straight edge so the R-core is much more linear. Because of this, it is capable of being used in pre-amp stages, chokes, and output stages. However, it still is not as easy winding the assembly as an EI type. It is somewhat hard to get balanced windings as each side has it's own. Yet, if it is properly manufactured with quality control, balance issues are non-existent. There is leakage (less than 1/10th of an EI type) however it is much more controlled and minimized by the design so some shielding is still recommended depending on placement of parts and type of circuit. R cores have been known to be cost prohibitive yet are common in higher quality equipment for power transformer, pre-amp, chokes, and outputs. A unique feature is that the bobbins are separate, isolating the two windings which make for a safer transformer.
So...there is the science lesson for today. What types do you have in your boomboxes? 
I can tell you this, the ones I have with the R and Torroidials...never have the lamps fade when cranked and the boomers never feel or sound starved. Absolutely not.
However, there are some manufacturers who will put a more expensive transformer in their product.
For example:
My Telefunken HiFi Studio-1, Hitachi J3, Telefunken CC9000 all use the highly regarded "R-Core" transformer. Of course the size of the transformer in the Studio-1 is ridiculous for obvious reasons.
And others like my Hitachi TRK-9900 use a torroidial transformer.
Typically what you will see in boomboxes are the following EI types...
I have (or have had) a unit that used a C-Core (I know I saw it but can' remember which it was.
)...In a good chunk of my "Heavy Hitters" the two former I see more than the latter. Here are the major differences with these types starting with the low cost massively used EI...
1. EI Type - It's made of an E-shape lamination and an "I" shaped lamination and very easy to wind accurately. Commonly used for more than 100 years due to the easy workability, it can be wound in very complex designs. But there is a cut-end on them and magnetic leakage loss is high. Interference and noise are high too. There are still many power transformers, choke, output and IT using this format as it is still the most cost effective to date.
2. C Core - A very rare but great core. There are double C cores and normal C-cores. Normal C-cores just looks like an R-core cut into 2 pieces. The cut edge is an air gap to prevent core saturation. It also allows the winding to be done externally. So, it has both advantages of EI and R-core. It has a slightly lower efficiency than R-cores but much better than EI. It was commonly used for many years in higher end equipment as an audio PT, choke, and OPT and choke. Making a C-core is much harder than a EI and therefore typically cost prohibitive for mass production on very large scales.
3. Torroidial - The core is a roll of long thin ferrous sheet. Since it has almost no cut edge, the magnetic loss is near zero. Also, noise and interference is very low. It can be placed very close to amplification parts without the need for heavy shielding. It is extremely suitable for compact designs as it also has very high efficiency and low heat generation. Because the windings are affected due to the inner core and outer core being a different size, the windings are not as accurate or linear and can affect sound in audio applications where used downstream in a circuit. Winding of torroidials is probably the most difficult of all the cores mentioned. Because of the issues with linearity these are used generally as power supplies mains only. Because there are no gaps in the windings torroidials are more susceptible to having their ability to pass some AC power limited by by residual DC currents in the windings.
4. R Core - These were developed by the Japanese as an improvement on the torroidial. Windings are only applied on the straight edge so the R-core is much more linear. Because of this, it is capable of being used in pre-amp stages, chokes, and output stages. However, it still is not as easy winding the assembly as an EI type. It is somewhat hard to get balanced windings as each side has it's own. Yet, if it is properly manufactured with quality control, balance issues are non-existent. There is leakage (less than 1/10th of an EI type) however it is much more controlled and minimized by the design so some shielding is still recommended depending on placement of parts and type of circuit. R cores have been known to be cost prohibitive yet are common in higher quality equipment for power transformer, pre-amp, chokes, and outputs. A unique feature is that the bobbins are separate, isolating the two windings which make for a safer transformer.
So...there is the science lesson for today.
What types do you have in your boomboxes? I can tell you this, the ones I have with the R and Torroidials...never have the lamps fade when cranked and the boomers never feel or sound starved. Absolutely not.
